diff --git a/.eslintrc.js b/.eslintrc.js
index 5d77acc57..3416e36a1 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -17,13 +17,6 @@ module.exports = {
     curly: "error",
     "dot-notation": "error",
     eqeqeq: "error",
-    "import/no-extraneous-dependencies": [
-      "error",
-      {
-        devDependencies: ["tests*/**", "scripts/**"],
-      },
-    ],
-    "import/order": "error",
     "no-console": isCI ? "error" : "off",
     "no-else-return": [
       "error",
@@ -96,6 +89,16 @@ module.exports = {
         exceptRange: true,
       },
     ],
+
+    "import/extensions": ["error", "always", { ignorePackages: true }],
+    "import/no-extraneous-dependencies": [
+      "error",
+      {
+        devDependencies: ["tests*/**", "scripts/**"],
+      },
+    ],
+    "import/order": "error",
+
     "regexp/match-any": [
       "error",
       {
@@ -103,6 +106,7 @@ module.exports = {
       },
     ],
     "regexp/no-useless-flag": "error",
+
     "unicorn/better-regex": "error",
     "unicorn/explicit-length-check": "error",
     "unicorn/new-for-builtins": "error",
diff --git a/bin/prettier.js b/bin/prettier.js
index 7f3f91780..afd811615 100755
--- a/bin/prettier.js
+++ b/bin/prettier.js
@@ -2,4 +2,4 @@
 
 "use strict";
 
-module.exports = require("../src/cli").run(process.argv.slice(2));
+module.exports = require("../src/cli/index.js").run(process.argv.slice(2));
diff --git a/index.js b/index.js
index f38df479e..dc0ab434b 100644
--- a/index.js
+++ b/index.js
@@ -1,3 +1,3 @@
 "use strict";
 
-module.exports = require("./src/index");
+module.exports = require("./src/index.js");
diff --git a/jest.config.js b/jest.config.js
index 684a50a5e..046c5e43e 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const path = require("path");
-const installPrettier = require("./scripts/install-prettier");
+const installPrettier = require("./scripts/install-prettier.js");
 
 const PROJECT_ROOT = __dirname;
 const isProduction = process.env.NODE_ENV === "production";
diff --git a/scripts/build/build.mjs b/scripts/build/build.mjs
index 4d1ff4dd2..4bcf89812 100644
--- a/scripts/build/build.mjs
+++ b/scripts/build/build.mjs
@@ -22,7 +22,7 @@ process.on("unhandledRejection", (err) => {
   process.exit(1);
 });
 
-const CACHE_VERSION = "v37"; // This need update when updating build scripts
+const CACHE_VERSION = "v38"; // This need update when updating build scripts
 const statusConfig = [
   { color: "bgYellow", text: "CACHED" },
   { color: "bgGreen", text: "DONE" },
diff --git a/scripts/release/release.js b/scripts/release/release.js
index 093ea240c..8a737a6b9 100644
--- a/scripts/release/release.js
+++ b/scripts/release/release.js
@@ -9,7 +9,7 @@ async function run() {
   const minimist = require("minimist");
   const semver = require("semver");
   const { string: outdentString } = require("outdent");
-  const { runGit, readJson } = require("./utils");
+  const { runGit, readJson } = require("./utils.js");
 
   const params = minimist(process.argv.slice(2), {
     string: ["version"],
@@ -33,18 +33,18 @@ async function run() {
   }
 
   const steps = [
-    require("./steps/validate-new-version"),
-    require("./steps/check-git-status"),
-    require("./steps/install-dependencies"),
-    require("./steps/run-tests"),
-    require("./steps/update-version"),
-    require("./steps/generate-bundles"),
-    require("./steps/update-changelog"),
-    require("./steps/push-to-git"),
-    require("./steps/publish-to-npm"),
-    require("./steps/bump-prettier"),
-    require("./steps/update-dependents-count"),
-    require("./steps/post-publish-steps"),
+    require("./steps/validate-new-version.js"),
+    require("./steps/check-git-status.js"),
+    require("./steps/install-dependencies.js"),
+    require("./steps/run-tests.js"),
+    require("./steps/update-version.js"),
+    require("./steps/generate-bundles.js"),
+    require("./steps/update-changelog.js"),
+    require("./steps/push-to-git.js"),
+    require("./steps/publish-to-npm.js"),
+    require("./steps/bump-prettier.js"),
+    require("./steps/update-dependents-count.js"),
+    require("./steps/post-publish-steps.js"),
   ];
 
   try {
diff --git a/scripts/release/steps/bump-prettier.js b/scripts/release/steps/bump-prettier.js
index 96a4ec160..1a64b1911 100644
--- a/scripts/release/steps/bump-prettier.js
+++ b/scripts/release/steps/bump-prettier.js
@@ -8,7 +8,7 @@ const {
   logPromise,
   readJson,
   writeJson,
-} = require("../utils");
+} = require("../utils.js");
 
 async function format() {
   await runYarn(["lint:eslint", "--fix"]);
diff --git a/scripts/release/steps/check-git-status.js b/scripts/release/steps/check-git-status.js
index 9c2106772..139098a9a 100644
--- a/scripts/release/steps/check-git-status.js
+++ b/scripts/release/steps/check-git-status.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { runGit } = require("../utils");
+const { runGit } = require("../utils.js");
 
 module.exports = async function () {
   const { stdout: status } = await runGit(["status", "--porcelain"]);
diff --git a/scripts/release/steps/generate-bundles.js b/scripts/release/steps/generate-bundles.js
index c2dd2c260..f52b77827 100644
--- a/scripts/release/steps/generate-bundles.js
+++ b/scripts/release/steps/generate-bundles.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const chalk = require("chalk");
-const { runYarn, logPromise, readJson } = require("../utils");
+const { runYarn, logPromise, readJson } = require("../utils.js");
 
 module.exports = async function ({ version }) {
   await logPromise("Generating bundles", runYarn(["build", "--purge-cache"]));
diff --git a/scripts/release/steps/install-dependencies.js b/scripts/release/steps/install-dependencies.js
index a304bcf79..62d156df3 100644
--- a/scripts/release/steps/install-dependencies.js
+++ b/scripts/release/steps/install-dependencies.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const execa = require("execa");
-const { runYarn, runGit, logPromise } = require("../utils");
+const { runYarn, runGit, logPromise } = require("../utils.js");
 
 async function install() {
   await execa("rm", ["-rf", "node_modules"]);
diff --git a/scripts/release/steps/post-publish-steps.js b/scripts/release/steps/post-publish-steps.js
index b87132e47..997cdccd2 100644
--- a/scripts/release/steps/post-publish-steps.js
+++ b/scripts/release/steps/post-publish-steps.js
@@ -3,7 +3,7 @@
 const chalk = require("chalk");
 const { string: outdentString } = require("outdent");
 const execa = require("execa");
-const { fetchText, logPromise } = require("../utils");
+const { fetchText, logPromise } = require("../utils.js");
 
 const SCHEMA_REPO = "SchemaStore/schemastore";
 const SCHEMA_PATH = "src/schemas/json/prettierrc.json";
diff --git a/scripts/release/steps/publish-to-npm.js b/scripts/release/steps/publish-to-npm.js
index 73db7fe5a..9e884b177 100644
--- a/scripts/release/steps/publish-to-npm.js
+++ b/scripts/release/steps/publish-to-npm.js
@@ -3,7 +3,7 @@
 const chalk = require("chalk");
 const { string: outdentString } = require("outdent");
 const execa = require("execa");
-const { logPromise, waitForEnter } = require("../utils");
+const { logPromise, waitForEnter } = require("../utils.js");
 
 /**
  * Retry "npm publish" when to enter OTP is failed.
diff --git a/scripts/release/steps/push-to-git.js b/scripts/release/steps/push-to-git.js
index e85f0612c..646fec422 100644
--- a/scripts/release/steps/push-to-git.js
+++ b/scripts/release/steps/push-to-git.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { runGit, logPromise } = require("../utils");
+const { runGit, logPromise } = require("../utils.js");
 
 async function pushGit({ version }) {
   await runGit(["commit", "-am", `Release ${version}`]);
diff --git a/scripts/release/steps/run-tests.js b/scripts/release/steps/run-tests.js
index 34315ed6b..84189ae78 100644
--- a/scripts/release/steps/run-tests.js
+++ b/scripts/release/steps/run-tests.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { runYarn, logPromise } = require("../utils");
+const { runYarn, logPromise } = require("../utils.js");
 
 module.exports = async function () {
   await logPromise("Running linter", runYarn("lint:eslint"));
diff --git a/scripts/release/steps/update-changelog.js b/scripts/release/steps/update-changelog.js
index fa9ba1849..afc6c997f 100644
--- a/scripts/release/steps/update-changelog.js
+++ b/scripts/release/steps/update-changelog.js
@@ -5,7 +5,7 @@ const execa = require("execa");
 const chalk = require("chalk");
 const { outdent, string: outdentString } = require("outdent");
 const semver = require("semver");
-const { waitForEnter, runYarn, logPromise } = require("../utils");
+const { waitForEnter, runYarn, logPromise } = require("../utils.js");
 
 function getBlogPostInfo(version) {
   const date = new Date();
diff --git a/scripts/release/steps/update-dependents-count.js b/scripts/release/steps/update-dependents-count.js
index 73ee3ae6f..43dd01a60 100644
--- a/scripts/release/steps/update-dependents-count.js
+++ b/scripts/release/steps/update-dependents-count.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const chalk = require("chalk");
-const { runGit, fetchText, logPromise, processFile } = require("../utils");
+const { runGit, fetchText, logPromise, processFile } = require("../utils.js");
 
 async function update() {
   const npmPage = await logPromise(
diff --git a/scripts/release/steps/update-version.js b/scripts/release/steps/update-version.js
index 1df08da79..7e1b29ad9 100644
--- a/scripts/release/steps/update-version.js
+++ b/scripts/release/steps/update-version.js
@@ -6,7 +6,7 @@ const {
   readJson,
   writeJson,
   processFile,
-} = require("../utils");
+} = require("../utils.js");
 
 async function bump({ version }) {
   const pkg = await readJson("package.json");
diff --git a/scripts/tools/eslint-plugin-prettier-internal-rules/index.js b/scripts/tools/eslint-plugin-prettier-internal-rules/index.js
index 7f941fb01..0a98121bf 100644
--- a/scripts/tools/eslint-plugin-prettier-internal-rules/index.js
+++ b/scripts/tools/eslint-plugin-prettier-internal-rules/index.js
@@ -2,20 +2,20 @@
 
 module.exports = {
   rules: {
-    "better-parent-property-check-in-needs-parens": require("./better-parent-property-check-in-needs-parens"),
-    "consistent-negative-index-access": require("./consistent-negative-index-access"),
-    "directly-loc-start-end": require("./directly-loc-start-end"),
-    "flat-ast-path-call": require("./flat-ast-path-call"),
-    "jsx-identifier-case": require("./jsx-identifier-case"),
-    "no-conflicting-comment-check-flags": require("./no-conflicting-comment-check-flags"),
-    "no-doc-builder-concat": require("./no-doc-builder-concat"),
-    "no-empty-flat-contents-for-if-break": require("./no-empty-flat-contents-for-if-break"),
-    "no-identifier-n": require("./no-identifier-n"),
-    "no-node-comments": require("./no-node-comments"),
-    "no-unnecessary-ast-path-call": require("./no-unnecessary-ast-path-call"),
-    "prefer-ast-path-each": require("./prefer-ast-path-each"),
-    "prefer-indent-if-break": require("./prefer-indent-if-break"),
-    "prefer-is-non-empty-array": require("./prefer-is-non-empty-array"),
-    "require-json-extensions": require("./require-json-extensions"),
+    "better-parent-property-check-in-needs-parens": require("./better-parent-property-check-in-needs-parens.js"),
+    "consistent-negative-index-access": require("./consistent-negative-index-access.js"),
+    "directly-loc-start-end": require("./directly-loc-start-end.js"),
+    "flat-ast-path-call": require("./flat-ast-path-call.js"),
+    "jsx-identifier-case": require("./jsx-identifier-case.js"),
+    "no-conflicting-comment-check-flags": require("./no-conflicting-comment-check-flags.js"),
+    "no-doc-builder-concat": require("./no-doc-builder-concat.js"),
+    "no-empty-flat-contents-for-if-break": require("./no-empty-flat-contents-for-if-break.js"),
+    "no-identifier-n": require("./no-identifier-n.js"),
+    "no-node-comments": require("./no-node-comments.js"),
+    "no-unnecessary-ast-path-call": require("./no-unnecessary-ast-path-call.js"),
+    "prefer-ast-path-each": require("./prefer-ast-path-each.js"),
+    "prefer-indent-if-break": require("./prefer-indent-if-break.js"),
+    "prefer-is-non-empty-array": require("./prefer-is-non-empty-array.js"),
+    "require-json-extensions": require("./require-json-extensions.js"),
   },
 };
diff --git a/src/cli/constant.js b/src/cli/constant.js
index 52758d2e9..05f45556e 100644
--- a/src/cli/constant.js
+++ b/src/cli/constant.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const { outdent } = require("outdent");
-const { coreOptions } = require("./prettier-internal");
+const { coreOptions } = require("./prettier-internal.js");
 
 const categoryOrder = [
   coreOptions.CATEGORY_OUTPUT,
diff --git a/src/cli/context.js b/src/cli/context.js
index 284e30c2f..a2a869a4d 100644
--- a/src/cli/context.js
+++ b/src/cli/context.js
@@ -2,19 +2,19 @@
 const pick = require("lodash/pick");
 
 // eslint-disable-next-line no-restricted-modules
-const prettier = require("../index");
+const prettier = require("../index.js");
 const {
   optionsModule,
   optionsNormalizer: { normalizeCliOptions },
   utils: { arrayify },
-} = require("./prettier-internal");
-const minimist = require("./minimist");
-const constant = require("./constant");
+} = require("./prettier-internal.js");
+const minimist = require("./minimist.js");
+const constant = require("./constant.js");
 const {
   createDetailedOptionMap,
   normalizeDetailedOptionMap,
-} = require("./option-map");
-const createMinimistOptions = require("./create-minimist-options");
+} = require("./option-map.js");
+const createMinimistOptions = require("./create-minimist-options.js");
 
 /**
  * @typedef {Object} Context
diff --git a/src/cli/core.js b/src/cli/core.js
index 2b9ca19ed..5d7836f52 100644
--- a/src/cli/core.js
+++ b/src/cli/core.js
@@ -5,16 +5,16 @@ const path = require("path");
 const stringify = require("fast-json-stable-stringify");
 
 // eslint-disable-next-line no-restricted-modules
-const prettier = require("../index");
+const prettier = require("../index.js");
 
-const { format, formatStdin, formatFiles } = require("./format");
-const { Context, parseArgvWithoutPlugins } = require("./context");
+const { format, formatStdin, formatFiles } = require("./format.js");
+const { Context, parseArgvWithoutPlugins } = require("./context.js");
 const {
   normalizeDetailedOptionMap,
   createDetailedOptionMap,
-} = require("./option-map");
-const { createDetailedUsage, createUsage } = require("./usage");
-const { createLogger } = require("./logger");
+} = require("./option-map.js");
+const { createDetailedUsage, createUsage } = require("./usage.js");
+const { createLogger } = require("./logger.js");
 
 async function logResolvedConfigPathOrDie(context) {
   const file = context.argv["find-config-path"];
diff --git a/src/cli/format.js b/src/cli/format.js
index 93dfc3650..208066d8b 100644
--- a/src/cli/format.js
+++ b/src/cli/format.js
@@ -6,14 +6,14 @@ const path = require("path");
 const chalk = require("chalk");
 
 // eslint-disable-next-line no-restricted-modules
-const prettier = require("../index");
+const prettier = require("../index.js");
 // eslint-disable-next-line no-restricted-modules
-const { getStdin } = require("../common/third-party");
+const { getStdin } = require("../common/third-party.js");
 
-const { createIgnorer, errors } = require("./prettier-internal");
-const { expandPatterns, fixWindowsSlashes } = require("./expand-patterns");
-const { getOptionsForFile } = require("./option");
-const isTTY = require("./is-tty");
+const { createIgnorer, errors } = require("./prettier-internal.js");
+const { expandPatterns, fixWindowsSlashes } = require("./expand-patterns.js");
+const { getOptionsForFile } = require("./option.js");
+const isTTY = require("./is-tty.js");
 
 function diff(a, b) {
   return require("diff").createTwoFilesPatch("", "", a, b, "", "", {
diff --git a/src/cli/index.js b/src/cli/index.js
index bf858981b..15a47938d 100644
--- a/src/cli/index.js
+++ b/src/cli/index.js
@@ -7,8 +7,8 @@ require("please-upgrade-node")(packageJson);
 // eslint-disable-next-line import/order
 const stringify = require("fast-json-stable-stringify");
 // eslint-disable-next-line no-restricted-modules
-const prettier = require("../index");
-const core = require("./core");
+const prettier = require("../index.js");
+const core = require("./core.js");
 
 async function run(rawArguments) {
   // Create a default level logger, so we can log errors during `logLevel` parsing
diff --git a/src/cli/is-tty.js b/src/cli/is-tty.js
index 0eb7ed199..b3e10b71c 100644
--- a/src/cli/is-tty.js
+++ b/src/cli/is-tty.js
@@ -1,7 +1,7 @@
 "use strict";
 
 // eslint-disable-next-line no-restricted-modules
-const { isCI } = require("../common/third-party");
+const { isCI } = require("../common/third-party.js");
 
 // Some CI pipelines incorrectly report process.stdout.isTTY status,
 // which causes unwanted lines in the output. An additional check for isCI() helps.
diff --git a/src/cli/option-map.js b/src/cli/option-map.js
index 256512450..fa906c7fd 100644
--- a/src/cli/option-map.js
+++ b/src/cli/option-map.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const dashify = require("dashify");
-const { coreOptions } = require("./prettier-internal");
+const { coreOptions } = require("./prettier-internal.js");
 
 function normalizeDetailedOption(name, option) {
   return {
diff --git a/src/cli/option.js b/src/cli/option.js
index 13bd9662d..d66f42372 100644
--- a/src/cli/option.js
+++ b/src/cli/option.js
@@ -2,10 +2,10 @@
 
 const dashify = require("dashify");
 // eslint-disable-next-line no-restricted-modules
-const prettier = require("../index");
-const minimist = require("./minimist");
-const { optionsNormalizer } = require("./prettier-internal");
-const createMinimistOptions = require("./create-minimist-options");
+const prettier = require("../index.js");
+const minimist = require("./minimist.js");
+const { optionsNormalizer } = require("./prettier-internal.js");
+const createMinimistOptions = require("./create-minimist-options.js");
 
 function getOptions(argv, detailedOptions) {
   return Object.fromEntries(
diff --git a/src/cli/prettier-internal.js b/src/cli/prettier-internal.js
index 66f81d6dd..cee1851c9 100644
--- a/src/cli/prettier-internal.js
+++ b/src/cli/prettier-internal.js
@@ -1,4 +1,4 @@
 "use strict";
 
 // eslint-disable-next-line no-restricted-modules
-module.exports = require("../index").__internal;
+module.exports = require("../index.js").__internal;
diff --git a/src/cli/usage.js b/src/cli/usage.js
index 9ba5ab599..e1be6badd 100644
--- a/src/cli/usage.js
+++ b/src/cli/usage.js
@@ -2,7 +2,7 @@
 
 const groupBy = require("lodash/groupBy");
 const camelCase = require("camelcase");
-const constant = require("./constant");
+const constant = require("./constant.js");
 
 const OPTION_USAGE_THRESHOLD = 25;
 const CHOICE_USAGE_MARGIN = 3;
diff --git a/src/common/ast-path.js b/src/common/ast-path.js
index e9d5542f1..ade4952a1 100644
--- a/src/common/ast-path.js
+++ b/src/common/ast-path.js
@@ -1,5 +1,5 @@
 "use strict";
-const getLast = require("../utils/get-last");
+const getLast = require("../utils/get-last.js");
 
 function getNodeHelper(path, count) {
   const stackIndex = getNodeStackIndexHelper(path.stack, count);
diff --git a/src/common/create-ignorer.js b/src/common/create-ignorer.js
index 6d7ba77f4..677ca876a 100644
--- a/src/common/create-ignorer.js
+++ b/src/common/create-ignorer.js
@@ -2,7 +2,7 @@
 
 const path = require("path");
 const ignore = require("ignore");
-const getFileContentOrNull = require("../utils/get-file-content-or-null");
+const getFileContentOrNull = require("../utils/get-file-content-or-null.js");
 
 /**
  * @param {string?} ignorePath
diff --git a/src/common/get-file-info.js b/src/common/get-file-info.js
index f7f2776e6..d01d1ee95 100644
--- a/src/common/get-file-info.js
+++ b/src/common/get-file-info.js
@@ -1,9 +1,9 @@
 "use strict";
 
 const path = require("path");
-const options = require("../main/options");
-const config = require("../config/resolve-config");
-const createIgnorer = require("./create-ignorer");
+const options = require("../main/options.js");
+const config = require("../config/resolve-config.js");
+const createIgnorer = require("./create-ignorer.js");
 
 /**
  * @typedef {{ ignorePath?: string, withNodeModules?: boolean, plugins: object }} FileInfoOptions
diff --git a/src/common/load-plugins.js b/src/common/load-plugins.js
index 1e00466aa..933693b6d 100644
--- a/src/common/load-plugins.js
+++ b/src/common/load-plugins.js
@@ -6,9 +6,9 @@ const uniqBy = require("lodash/uniqBy");
 const partition = require("lodash/partition");
 const globby = require("globby");
 const mem = require("mem");
-const internalPlugins = require("../languages");
-const thirdParty = require("./third-party");
-const resolve = require("./resolve");
+const internalPlugins = require("../languages.js");
+const thirdParty = require("./third-party.js");
+const resolve = require("./resolve.js");
 
 const memoizedLoad = mem(load, { cacheKey: JSON.stringify });
 const memoizedSearch = mem(findPluginsInNodeModules);
diff --git a/src/common/util-shared.js b/src/common/util-shared.js
index 05120acdd..0c03bb731 100644
--- a/src/common/util-shared.js
+++ b/src/common/util-shared.js
@@ -24,7 +24,7 @@ const {
   addLeadingComment,
   addDanglingComment,
   addTrailingComment,
-} = require("./util");
+} = require("./util.js");
 
 module.exports = {
   getMaxContinuousCount,
diff --git a/src/common/util.js b/src/common/util.js
index 94b8591c7..4e200b418 100644
--- a/src/common/util.js
+++ b/src/common/util.js
@@ -2,8 +2,8 @@
 
 const stringWidth = require("string-width");
 const escapeStringRegexp = require("escape-string-regexp");
-const getLast = require("../utils/get-last");
-const { getSupportInfo } = require("../main/support");
+const getLast = require("../utils/get-last.js");
+const { getSupportInfo } = require("../main/support.js");
 
 const notAsciiRegex = /[^\x20-\x7F]/;
 
diff --git a/src/config/resolve-config-editorconfig.js b/src/config/resolve-config-editorconfig.js
index ebf997123..06105b44d 100644
--- a/src/config/resolve-config-editorconfig.js
+++ b/src/config/resolve-config-editorconfig.js
@@ -5,7 +5,7 @@ const path = require("path");
 const editorconfig = require("editorconfig");
 const mem = require("mem");
 const editorConfigToPrettier = require("editorconfig-to-prettier");
-const findProjectRoot = require("./find-project-root");
+const findProjectRoot = require("./find-project-root.js");
 
 const jsonStringifyMem = (fn) => mem(fn, { cacheKey: JSON.stringify });
 
diff --git a/src/config/resolve-config.js b/src/config/resolve-config.js
index c2cac666c..edcf0108a 100644
--- a/src/config/resolve-config.js
+++ b/src/config/resolve-config.js
@@ -3,12 +3,12 @@
 const path = require("path");
 const minimatch = require("minimatch");
 const mem = require("mem");
-const thirdParty = require("../common/third-party");
+const thirdParty = require("../common/third-party.js");
 
-const loadToml = require("../utils/load-toml");
-const loadJson5 = require("../utils/load-json5");
-const resolve = require("../common/resolve");
-const resolveEditorConfig = require("./resolve-config-editorconfig");
+const loadToml = require("../utils/load-toml.js");
+const loadJson5 = require("../utils/load-json5.js");
+const resolve = require("../common/resolve.js");
+const resolveEditorConfig = require("./resolve-config-editorconfig.js");
 
 /**
  * @typedef {import("cosmiconfig/dist/Explorer").Explorer} Explorer
diff --git a/src/document/doc-debug.js b/src/document/doc-debug.js
index 6d7050929..02a65e436 100644
--- a/src/document/doc-debug.js
+++ b/src/document/doc-debug.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { isConcat, getDocParts } = require("./doc-utils");
+const { isConcat, getDocParts } = require("./doc-utils.js");
 
 function flattenDoc(doc) {
   if (!doc) {
diff --git a/src/document/doc-printer.js b/src/document/doc-printer.js
index 5ebf6c579..4b36f4093 100644
--- a/src/document/doc-printer.js
+++ b/src/document/doc-printer.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const { getStringWidth, getLast } = require("../common/util");
-const { convertEndOfLineToChars } = require("../common/end-of-line");
-const { fill, cursor, indent } = require("./doc-builders");
-const { isConcat, getDocParts } = require("./doc-utils");
+const { getStringWidth, getLast } = require("../common/util.js");
+const { convertEndOfLineToChars } = require("../common/end-of-line.js");
+const { fill, cursor, indent } = require("./doc-builders.js");
+const { isConcat, getDocParts } = require("./doc-utils.js");
 
 /** @type {Record<symbol, typeof MODE_BREAK | typeof MODE_FLAT>} */
 let groupModeMap;
diff --git a/src/document/doc-utils.js b/src/document/doc-utils.js
index 4faec9473..63be7f6f6 100644
--- a/src/document/doc-utils.js
+++ b/src/document/doc-utils.js
@@ -1,6 +1,6 @@
 "use strict";
-const getLast = require("../utils/get-last");
-const { literalline, join } = require("./doc-builders");
+const getLast = require("../utils/get-last.js");
+const { literalline, join } = require("./doc-builders.js");
 
 const isConcat = (doc) => Array.isArray(doc) || (doc && doc.type === "concat");
 const getDocParts = (doc) => {
diff --git a/src/document/index.js b/src/document/index.js
index 3629a9c21..03eebf216 100644
--- a/src/document/index.js
+++ b/src/document/index.js
@@ -5,8 +5,8 @@
  */
 
 module.exports = {
-  builders: require("./doc-builders"),
-  printer: require("./doc-printer"),
-  utils: require("./doc-utils"),
-  debug: require("./doc-debug"),
+  builders: require("./doc-builders.js"),
+  printer: require("./doc-printer.js"),
+  utils: require("./doc-utils.js"),
+  debug: require("./doc-debug.js"),
 };
diff --git a/src/index.js b/src/index.js
index 143b39934..e091aeae2 100644
--- a/src/index.js
+++ b/src/index.js
@@ -2,13 +2,13 @@
 
 const { version } = require("../package.json");
 
-const core = require("./main/core");
-const { getSupportInfo } = require("./main/support");
-const getFileInfo = require("./common/get-file-info");
-const sharedUtil = require("./common/util-shared");
-const plugins = require("./common/load-plugins");
-const config = require("./config/resolve-config");
-const doc = require("./document");
+const core = require("./main/core.js");
+const { getSupportInfo } = require("./main/support.js");
+const getFileInfo = require("./common/get-file-info.js");
+const sharedUtil = require("./common/util-shared.js");
+const plugins = require("./common/load-plugins.js");
+const config = require("./config/resolve-config.js");
+const doc = require("./document/index.js");
 
 function _withPlugins(
   fn,
@@ -67,13 +67,13 @@ module.exports = {
 
   // Internal shared
   __internal: {
-    errors: require("./common/errors"),
-    coreOptions: require("./main/core-options"),
-    createIgnorer: require("./common/create-ignorer"),
-    optionsModule: require("./main/options"),
-    optionsNormalizer: require("./main/options-normalizer"),
+    errors: require("./common/errors.js"),
+    coreOptions: require("./main/core-options.js"),
+    createIgnorer: require("./common/create-ignorer.js"),
+    optionsModule: require("./main/options.js"),
+    optionsNormalizer: require("./main/options-normalizer.js"),
     utils: {
-      arrayify: require("./utils/arrayify"),
+      arrayify: require("./utils/arrayify.js"),
     },
   },
 
diff --git a/src/language-css/clean.js b/src/language-css/clean.js
index 3e103456c..372f0069f 100644
--- a/src/language-css/clean.js
+++ b/src/language-css/clean.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const { isFrontMatterNode } = require("../common/util");
-const getLast = require("../utils/get-last");
+const { isFrontMatterNode } = require("../common/util.js");
+const getLast = require("../utils/get-last.js");
 
 const ignoredProperties = new Set([
   "raw", // front-matter
diff --git a/src/language-css/embed.js b/src/language-css/embed.js
index 19c7371c3..1f96780b4 100644
--- a/src/language-css/embed.js
+++ b/src/language-css/embed.js
@@ -1,8 +1,8 @@
 "use strict";
 const {
   builders: { hardline },
-} = require("../document");
-const printFrontMatter = require("../utils/front-matter/print");
+} = require("../document/index.js");
+const printFrontMatter = require("../utils/front-matter/print.js");
 
 function embed(path, print, textToDoc /*, options */) {
   const node = path.getValue();
diff --git a/src/language-css/index.js b/src/language-css/index.js
index 01deeb7df..75f63f98f 100644
--- a/src/language-css/index.js
+++ b/src/language-css/index.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const createLanguage = require("../utils/create-language");
-const printer = require("./printer-postcss");
-const options = require("./options");
-const parsers = require("./parsers");
+const createLanguage = require("../utils/create-language.js");
+const printer = require("./printer-postcss.js");
+const options = require("./options.js");
+const parsers = require("./parsers.js");
 
 const languages = [
   createLanguage(require("linguist-languages/data/CSS.json"), (data) => ({
diff --git a/src/language-css/loc.js b/src/language-css/loc.js
index 3562df36b..5c077301a 100644
--- a/src/language-css/loc.js
+++ b/src/language-css/loc.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const lineColumnToIndex = require("../utils/line-column-to-index");
-const { getLast, skipEverythingButNewLine } = require("../common/util");
+const lineColumnToIndex = require("../utils/line-column-to-index.js");
+const { getLast, skipEverythingButNewLine } = require("../common/util.js");
 
 function calculateLocStart(node, text) {
   // value-* nodes have this
diff --git a/src/language-css/options.js b/src/language-css/options.js
index 357fa26f2..f5f381ae9 100644
--- a/src/language-css/options.js
+++ b/src/language-css/options.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const commonOptions = require("../common/common-options");
+const commonOptions = require("../common/common-options.js");
 
 // format based on https://github.com/prettier/prettier/blob/main/src/main/core-options.js
 module.exports = {
diff --git a/src/language-css/parser-postcss.js b/src/language-css/parser-postcss.js
index 213e6f91c..71b466bc9 100644
--- a/src/language-css/parser-postcss.js
+++ b/src/language-css/parser-postcss.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const createError = require("../common/parser-create-error");
-const getLast = require("../utils/get-last");
-const parseFrontMatter = require("../utils/front-matter/parse");
-const { hasPragma } = require("./pragma");
+const createError = require("../common/parser-create-error.js");
+const getLast = require("../utils/get-last.js");
+const parseFrontMatter = require("../utils/front-matter/parse.js");
+const { hasPragma } = require("./pragma.js");
 const {
   hasSCSSInterpolation,
   hasStringOrFunction,
@@ -12,9 +12,9 @@ const {
   isSCSSNestedPropertyNode,
   isSCSSVariable,
   stringifyNode,
-} = require("./utils");
-const { locStart, locEnd } = require("./loc");
-const { calculateLoc, replaceQuotesInInlineComments } = require("./loc");
+} = require("./utils.js");
+const { locStart, locEnd } = require("./loc.js");
+const { calculateLoc, replaceQuotesInInlineComments } = require("./loc.js");
 
 const getHighestAncestor = (node) => {
   while (node.parent) {
diff --git a/src/language-css/parsers.js b/src/language-css/parsers.js
index d29892985..855020041 100644
--- a/src/language-css/parsers.js
+++ b/src/language-css/parsers.js
@@ -3,12 +3,12 @@
 module.exports = {
   // TODO: switch these to just `postcss` and use `language` instead.
   get css() {
-    return require("./parser-postcss").parsers.css;
+    return require("./parser-postcss.js").parsers.css;
   },
   get less() {
-    return require("./parser-postcss").parsers.less;
+    return require("./parser-postcss.js").parsers.less;
   },
   get scss() {
-    return require("./parser-postcss").parsers.scss;
+    return require("./parser-postcss.js").parsers.scss;
   },
 };
diff --git a/src/language-css/pragma.js b/src/language-css/pragma.js
index 872f50d9c..407e9f554 100644
--- a/src/language-css/pragma.js
+++ b/src/language-css/pragma.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const jsPragma = require("../language-js/pragma");
-const parseFrontMatter = require("../utils/front-matter/parse");
+const jsPragma = require("../language-js/pragma.js");
+const parseFrontMatter = require("../utils/front-matter/parse.js");
 
 function hasPragma(text) {
   return jsPragma.hasPragma(parseFrontMatter(text).content);
diff --git a/src/language-css/printer-postcss.js b/src/language-css/printer-postcss.js
index b2e79a477..c7ebe7e56 100644
--- a/src/language-css/printer-postcss.js
+++ b/src/language-css/printer-postcss.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const getLast = require("../utils/get-last");
+const getLast = require("../utils/get-last.js");
 const {
   printNumber,
   printString,
@@ -8,7 +8,7 @@ const {
   isFrontMatterNode,
   isNextLineEmpty,
   isNonEmptyArray,
-} = require("../common/util");
+} = require("../common/util.js");
 const {
   builders: {
     join,
@@ -23,10 +23,10 @@ const {
     breakParent,
   },
   utils: { removeLines, getDocParts },
-} = require("../document");
-const clean = require("./clean");
-const embed = require("./embed");
-const { insertPragma } = require("./pragma");
+} = require("../document/index.js");
+const clean = require("./clean.js");
+const embed = require("./embed.js");
+const { insertPragma } = require("./pragma.js");
 
 const {
   getAncestorNode,
@@ -74,8 +74,8 @@ const {
   isColorAdjusterFuncNode,
   lastLineHasInlineComment,
   isAtWordPlaceholderNode,
-} = require("./utils");
-const { locStart, locEnd } = require("./loc");
+} = require("./utils.js");
+const { locStart, locEnd } = require("./loc.js");
 
 function shouldPrintComma(options) {
   return options.trailingComma === "es5" || options.trailingComma === "all";
diff --git a/src/language-css/utils.js b/src/language-css/utils.js
index 62e38a4fe..cb40d132f 100644
--- a/src/language-css/utils.js
+++ b/src/language-css/utils.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { isNonEmptyArray } = require("../common/util");
+const { isNonEmptyArray } = require("../common/util.js");
 const colorAdjusterFunctions = new Set([
   "red",
   "green",
diff --git a/src/language-graphql/index.js b/src/language-graphql/index.js
index 365349de3..92e13c140 100644
--- a/src/language-graphql/index.js
+++ b/src/language-graphql/index.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const createLanguage = require("../utils/create-language");
-const printer = require("./printer-graphql");
-const options = require("./options");
-const parsers = require("./parsers");
+const createLanguage = require("../utils/create-language.js");
+const printer = require("./printer-graphql.js");
+const options = require("./options.js");
+const parsers = require("./parsers.js");
 
 const languages = [
   createLanguage(require("linguist-languages/data/GraphQL.json"), () => ({
diff --git a/src/language-graphql/options.js b/src/language-graphql/options.js
index cfe076dd4..fbef24d9f 100644
--- a/src/language-graphql/options.js
+++ b/src/language-graphql/options.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const commonOptions = require("../common/common-options");
+const commonOptions = require("../common/common-options.js");
 
 // format based on https://github.com/prettier/prettier/blob/main/src/main/core-options.js
 module.exports = {
diff --git a/src/language-graphql/parser-graphql.js b/src/language-graphql/parser-graphql.js
index 4b7ef8f5c..5947a7431 100644
--- a/src/language-graphql/parser-graphql.js
+++ b/src/language-graphql/parser-graphql.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const createError = require("../common/parser-create-error");
-const tryCombinations = require("../utils/try-combinations");
-const { hasPragma } = require("./pragma");
-const { locStart, locEnd } = require("./loc");
+const createError = require("../common/parser-create-error.js");
+const tryCombinations = require("../utils/try-combinations.js");
+const { hasPragma } = require("./pragma.js");
+const { locStart, locEnd } = require("./loc.js");
 
 function parseComments(ast) {
   const comments = [];
diff --git a/src/language-graphql/parsers.js b/src/language-graphql/parsers.js
index 9e7174c32..0bec48b6d 100644
--- a/src/language-graphql/parsers.js
+++ b/src/language-graphql/parsers.js
@@ -2,6 +2,6 @@
 
 module.exports = {
   get graphql() {
-    return require("./parser-graphql").parsers.graphql;
+    return require("./parser-graphql.js").parsers.graphql;
   },
 };
diff --git a/src/language-graphql/printer-graphql.js b/src/language-graphql/printer-graphql.js
index 4113999a0..74246e778 100644
--- a/src/language-graphql/printer-graphql.js
+++ b/src/language-graphql/printer-graphql.js
@@ -2,10 +2,10 @@
 
 const {
   builders: { join, hardline, line, softline, group, indent, ifBreak },
-} = require("../document");
-const { isNextLineEmpty, isNonEmptyArray } = require("../common/util");
-const { insertPragma } = require("./pragma");
-const { locStart, locEnd } = require("./loc");
+} = require("../document/index.js");
+const { isNextLineEmpty, isNonEmptyArray } = require("../common/util.js");
+const { insertPragma } = require("./pragma.js");
+const { locStart, locEnd } = require("./loc.js");
 
 function genericPrint(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-handlebars/index.js b/src/language-handlebars/index.js
index 437105f2d..f8ceac1ee 100644
--- a/src/language-handlebars/index.js
+++ b/src/language-handlebars/index.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const createLanguage = require("../utils/create-language");
-const printer = require("./printer-glimmer");
-const parsers = require("./parsers");
+const createLanguage = require("../utils/create-language.js");
+const printer = require("./printer-glimmer.js");
+const parsers = require("./parsers.js");
 
 const languages = [
   createLanguage(require("linguist-languages/data/Handlebars.json"), () => ({
diff --git a/src/language-handlebars/parser-glimmer.js b/src/language-handlebars/parser-glimmer.js
index 3df6859c3..dd223f491 100644
--- a/src/language-handlebars/parser-glimmer.js
+++ b/src/language-handlebars/parser-glimmer.js
@@ -1,8 +1,8 @@
 "use strict";
 
 const LinesAndColumns = require("lines-and-columns").default;
-const createError = require("../common/parser-create-error");
-const { locStart, locEnd } = require("./loc");
+const createError = require("../common/parser-create-error.js");
+const { locStart, locEnd } = require("./loc.js");
 
 /* from the following template: `non-escaped mustache \\{{helper}}`
  * glimmer parser will produce an AST missing a backslash
diff --git a/src/language-handlebars/parsers.js b/src/language-handlebars/parsers.js
index faeb17a87..5ae762496 100644
--- a/src/language-handlebars/parsers.js
+++ b/src/language-handlebars/parsers.js
@@ -2,6 +2,6 @@
 
 module.exports = {
   get glimmer() {
-    return require("./parser-glimmer").parsers.glimmer;
+    return require("./parser-glimmer.js").parsers.glimmer;
   },
 };
diff --git a/src/language-handlebars/printer-glimmer.js b/src/language-handlebars/printer-glimmer.js
index 66e7ae6e0..11ca713b9 100644
--- a/src/language-handlebars/printer-glimmer.js
+++ b/src/language-handlebars/printer-glimmer.js
@@ -14,10 +14,10 @@ const {
     literalline,
   },
   utils: { getDocParts, replaceEndOfLineWith },
-} = require("../document");
-const { isNonEmptyArray } = require("../common/util");
-const { locStart, locEnd } = require("./loc");
-const clean = require("./clean");
+} = require("../document/index.js");
+const { isNonEmptyArray } = require("../common/util.js");
+const { locStart, locEnd } = require("./loc.js");
+const clean = require("./clean.js");
 const {
   getNextNode,
   getPreviousNode,
@@ -29,7 +29,7 @@ const {
   isPreviousNodeOfSomeType,
   isVoid,
   isWhitespaceNode,
-} = require("./utils");
+} = require("./utils.js");
 
 const NEWLINES_TO_PRESERVE_MAX = 2;
 
diff --git a/src/language-handlebars/utils.js b/src/language-handlebars/utils.js
index 807a8cc2a..d4831d6f1 100644
--- a/src/language-handlebars/utils.js
+++ b/src/language-handlebars/utils.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const htmlVoidElements = require("html-void-elements");
-const getLast = require("../utils/get-last");
+const getLast = require("../utils/get-last.js");
 
 function isLastNodeOfSiblings(path) {
   const node = path.getValue();
diff --git a/src/language-html/ast.js b/src/language-html/ast.js
index fe7d38c80..a11fa691f 100644
--- a/src/language-html/ast.js
+++ b/src/language-html/ast.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const { isNonEmptyArray } = require("../common/util");
-const getLast = require("../utils/get-last");
+const { isNonEmptyArray } = require("../common/util.js");
+const getLast = require("../utils/get-last.js");
 
 const NODES_KEYS = {
   attrs: true,
diff --git a/src/language-html/clean.js b/src/language-html/clean.js
index 4c9089cc9..fbfc73f2e 100644
--- a/src/language-html/clean.js
+++ b/src/language-html/clean.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { isFrontMatterNode } = require("../common/util");
+const { isFrontMatterNode } = require("../common/util.js");
 
 const ignoredProperties = new Set([
   "sourceSpan",
diff --git a/src/language-html/index.js b/src/language-html/index.js
index 175548ad9..568f14254 100644
--- a/src/language-html/index.js
+++ b/src/language-html/index.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const createLanguage = require("../utils/create-language");
-const printer = require("./printer-html");
-const options = require("./options");
-const parsers = require("./parsers");
+const createLanguage = require("../utils/create-language.js");
+const printer = require("./printer-html.js");
+const options = require("./options.js");
+const parsers = require("./parsers.js");
 
 const languages = [
   createLanguage(require("linguist-languages/data/HTML.json"), () => ({
diff --git a/src/language-html/parser-html.js b/src/language-html/parser-html.js
index 3cd533c4f..13c2ec9a1 100644
--- a/src/language-html/parser-html.js
+++ b/src/language-html/parser-html.js
@@ -5,19 +5,19 @@ const {
   ParseLocation,
   ParseSourceFile,
 } = require("angular-html-parser/lib/compiler/src/parse_util");
-const parseFrontMatter = require("../utils/front-matter/parse");
-const getLast = require("../utils/get-last");
-const createError = require("../common/parser-create-error");
-const { inferParserByLanguage } = require("../common/util");
+const parseFrontMatter = require("../utils/front-matter/parse.js");
+const getLast = require("../utils/get-last.js");
+const createError = require("../common/parser-create-error.js");
+const { inferParserByLanguage } = require("../common/util.js");
 const {
   HTML_ELEMENT_ATTRIBUTES,
   HTML_TAGS,
   isUnknownNamespace,
-} = require("./utils");
-const { hasPragma } = require("./pragma");
-const { Node } = require("./ast");
-const { parseIeConditionalComment } = require("./conditional-comment");
-const { locStart, locEnd } = require("./loc");
+} = require("./utils.js");
+const { hasPragma } = require("./pragma.js");
+const { Node } = require("./ast.js");
+const { parseIeConditionalComment } = require("./conditional-comment.js");
+const { locStart, locEnd } = require("./loc.js");
 
 /**
  * @typedef {import('angular-html-parser/lib/compiler/src/ml_parser/ast').Node} AstNode
diff --git a/src/language-html/parsers.js b/src/language-html/parsers.js
index b775d9c47..5ff1998d8 100644
--- a/src/language-html/parsers.js
+++ b/src/language-html/parsers.js
@@ -3,18 +3,18 @@
 module.exports = {
   // HTML
   get html() {
-    return require("./parser-html").parsers.html;
+    return require("./parser-html.js").parsers.html;
   },
   // Vue
   get vue() {
-    return require("./parser-html").parsers.vue;
+    return require("./parser-html.js").parsers.vue;
   },
   // Angular
   get angular() {
-    return require("./parser-html").parsers.angular;
+    return require("./parser-html.js").parsers.angular;
   },
   // Lightning Web Components
   get lwc() {
-    return require("./parser-html").parsers.lwc;
+    return require("./parser-html.js").parsers.lwc;
   },
 };
diff --git a/src/language-html/print-preprocess.js b/src/language-html/print-preprocess.js
index 66545bba9..c38d0f0ec 100644
--- a/src/language-html/print-preprocess.js
+++ b/src/language-html/print-preprocess.js
@@ -3,7 +3,7 @@
 const {
   ParseSourceSpan,
 } = require("angular-html-parser/lib/compiler/src/parse_util");
-const getLast = require("../utils/get-last");
+const getLast = require("../utils/get-last.js");
 const {
   htmlTrim,
   getLeadingAndTrailingHtmlWhitespace,
@@ -15,7 +15,7 @@ const {
   isLeadingSpaceSensitiveNode,
   isTrailingSpaceSensitiveNode,
   isWhitespaceSensitiveNode,
-} = require("./utils");
+} = require("./utils.js");
 
 const PREPROCESS_PIPELINE = [
   removeIgnorableFirstLf,
diff --git a/src/language-html/printer-html.js b/src/language-html/printer-html.js
index 40b928319..eb883fdac 100644
--- a/src/language-html/printer-html.js
+++ b/src/language-html/printer-html.js
@@ -22,10 +22,10 @@ const {
     softline,
   },
   utils: { mapDoc, cleanDoc, getDocParts, isConcat, replaceEndOfLineWith },
-} = require("../document");
-const { isNonEmptyArray } = require("../common/util");
-const printFrontMatter = require("../utils/front-matter/print");
-const clean = require("./clean");
+} = require("../document/index.js");
+const { isNonEmptyArray } = require("../common/util.js");
+const printFrontMatter = require("../utils/front-matter/print.js");
+const clean = require("./clean.js");
 const {
   htmlTrimPreserveIndentation,
   splitByHtmlWhitespace,
@@ -50,16 +50,16 @@ const {
   shouldPreserveContent,
   unescapeQuoteEntities,
   isPreLikeNode,
-} = require("./utils");
-const preprocess = require("./print-preprocess");
-const { insertPragma } = require("./pragma");
-const { locStart, locEnd } = require("./loc");
+} = require("./utils.js");
+const preprocess = require("./print-preprocess.js");
+const { insertPragma } = require("./pragma.js");
+const { locStart, locEnd } = require("./loc.js");
 const {
   printVueFor,
   printVueBindings,
   isVueEventBindingExpression,
-} = require("./syntax-vue");
-const { printImgSrcset, printClassNames } = require("./syntax-attribute");
+} = require("./syntax-vue.js");
+const { printImgSrcset, printClassNames } = require("./syntax-attribute.js");
 
 function embed(path, print, textToDoc, options) {
   const node = path.getValue();
diff --git a/src/language-html/syntax-attribute.js b/src/language-html/syntax-attribute.js
index c7caa0a8a..130d77855 100644
--- a/src/language-html/syntax-attribute.js
+++ b/src/language-html/syntax-attribute.js
@@ -1,10 +1,10 @@
 "use strict";
 
 const parseSrcset = require("parse-srcset");
-const getLast = require("../utils/get-last");
+const getLast = require("../utils/get-last.js");
 const {
   builders: { group, ifBreak, indent, join, line, softline },
-} = require("../document");
+} = require("../document/index.js");
 
 function printImgSrcset(value) {
   const srcset = parseSrcset(value, {
diff --git a/src/language-html/syntax-vue.js b/src/language-html/syntax-vue.js
index df31e0f99..f029938dd 100644
--- a/src/language-html/syntax-vue.js
+++ b/src/language-html/syntax-vue.js
@@ -2,7 +2,7 @@
 
 const {
   builders: { group },
-} = require("../document");
+} = require("../document/index.js");
 
 /**
  *     v-for="... in ..."
diff --git a/src/language-html/utils.js b/src/language-html/utils.js
index f6d5a71aa..b16aff4cb 100644
--- a/src/language-html/utils.js
+++ b/src/language-html/utils.js
@@ -6,13 +6,16 @@
 
 const htmlTagNames = require("html-tag-names");
 const htmlElementAttributes = require("html-element-attributes");
-const { inferParserByLanguage, isFrontMatterNode } = require("../common/util");
+const {
+  inferParserByLanguage,
+  isFrontMatterNode,
+} = require("../common/util.js");
 const {
   CSS_DISPLAY_TAGS,
   CSS_DISPLAY_DEFAULT,
   CSS_WHITE_SPACE_TAGS,
   CSS_WHITE_SPACE_DEFAULT,
-} = require("./constants.evaluate");
+} = require("./constants.evaluate.js");
 
 const HTML_TAGS = arrayToMap(htmlTagNames);
 const HTML_ELEMENT_ATTRIBUTES = mapObject(htmlElementAttributes, arrayToMap);
diff --git a/src/language-js/clean.js b/src/language-js/clean.js
index cd67b0e35..00d13a482 100644
--- a/src/language-js/clean.js
+++ b/src/language-js/clean.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { isBlockComment } = require("./utils");
+const { isBlockComment } = require("./utils.js");
 
 const ignoredProperties = new Set([
   "range",
diff --git a/src/language-js/comments.js b/src/language-js/comments.js
index db830e2c6..fdc4050f9 100644
--- a/src/language-js/comments.js
+++ b/src/language-js/comments.js
@@ -11,7 +11,7 @@ const {
   addDanglingComment,
   getNextNonSpaceNonCommentCharacterIndex,
   isNonEmptyArray,
-} = require("../common/util");
+} = require("../common/util.js");
 const {
   isBlockComment,
   getFunctionParameters,
@@ -25,8 +25,8 @@ const {
   isCallExpression,
   isMemberExpression,
   isObjectProperty,
-} = require("./utils");
-const { locStart, locEnd } = require("./loc");
+} = require("./utils.js");
+const { locStart, locEnd } = require("./loc.js");
 
 /**
  * @typedef {import("./types/estree").Node} Node
diff --git a/src/language-js/embed.js b/src/language-js/embed.js
index 6b185e594..972a90af1 100644
--- a/src/language-js/embed.js
+++ b/src/language-js/embed.js
@@ -1,10 +1,14 @@
 "use strict";
 
-const { hasComment, CommentCheckFlags, isObjectProperty } = require("./utils");
-const formatMarkdown = require("./embed/markdown");
-const formatCss = require("./embed/css");
-const formatGraphql = require("./embed/graphql");
-const formatHtml = require("./embed/html");
+const {
+  hasComment,
+  CommentCheckFlags,
+  isObjectProperty,
+} = require("./utils.js");
+const formatMarkdown = require("./embed/markdown.js");
+const formatCss = require("./embed/css.js");
+const formatGraphql = require("./embed/graphql.js");
+const formatHtml = require("./embed/html.js");
 
 function getLanguage(path) {
   if (
diff --git a/src/language-js/embed/css.js b/src/language-js/embed/css.js
index 7ff50144c..89851014b 100644
--- a/src/language-js/embed/css.js
+++ b/src/language-js/embed/css.js
@@ -1,11 +1,11 @@
 "use strict";
 
-const { isNonEmptyArray } = require("../../common/util");
+const { isNonEmptyArray } = require("../../common/util.js");
 const {
   builders: { indent, hardline, softline },
   utils: { mapDoc, replaceNewlinesWithLiterallines, cleanDoc },
-} = require("../../document");
-const { printTemplateExpressions } = require("../print/template-literal");
+} = require("../../document/index.js");
+const { printTemplateExpressions } = require("../print/template-literal.js");
 
 function format(path, print, textToDoc) {
   const node = path.getValue();
diff --git a/src/language-js/embed/graphql.js b/src/language-js/embed/graphql.js
index 80d9d4764..24342ba76 100644
--- a/src/language-js/embed/graphql.js
+++ b/src/language-js/embed/graphql.js
@@ -2,11 +2,11 @@
 
 const {
   builders: { indent, join, hardline },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   escapeTemplateCharacters,
   printTemplateExpressions,
-} = require("../print/template-literal");
+} = require("../print/template-literal.js");
 
 function format(path, print, textToDoc) {
   const node = path.getValue();
diff --git a/src/language-js/embed/html.js b/src/language-js/embed/html.js
index 99f5d9bde..8913f803d 100644
--- a/src/language-js/embed/html.js
+++ b/src/language-js/embed/html.js
@@ -3,11 +3,11 @@
 const {
   builders: { indent, line, hardline, group },
   utils: { mapDoc },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   printTemplateExpressions,
   uncookTemplateElementValue,
-} = require("../print/template-literal");
+} = require("../print/template-literal.js");
 
 // The counter is needed to distinguish nested embeds.
 let htmlTemplateLiteralCounter = 0;
diff --git a/src/language-js/embed/markdown.js b/src/language-js/embed/markdown.js
index 2edf5471a..416d877a3 100644
--- a/src/language-js/embed/markdown.js
+++ b/src/language-js/embed/markdown.js
@@ -2,8 +2,8 @@
 
 const {
   builders: { indent, softline, literalline, dedentToRoot },
-} = require("../../document");
-const { escapeTemplateCharacters } = require("../print/template-literal");
+} = require("../../document/index.js");
+const { escapeTemplateCharacters } = require("../print/template-literal.js");
 
 function format(path, print, textToDoc) {
   const node = path.getValue();
diff --git a/src/language-js/index.js b/src/language-js/index.js
index 7a18e6de7..7d26529da 100644
--- a/src/language-js/index.js
+++ b/src/language-js/index.js
@@ -1,10 +1,10 @@
 "use strict";
 
-const createLanguage = require("../utils/create-language");
-const estreePrinter = require("./printer-estree");
-const estreeJsonPrinter = require("./printer-estree-json");
-const options = require("./options");
-const parsers = require("./parse/parsers");
+const createLanguage = require("../utils/create-language.js");
+const estreePrinter = require("./printer-estree.js");
+const estreeJsonPrinter = require("./printer-estree-json.js");
+const options = require("./options.js");
+const parsers = require("./parse/parsers.js");
 
 const languages = [
   createLanguage(
diff --git a/src/language-js/loc.js b/src/language-js/loc.js
index 080650428..969025b5a 100644
--- a/src/language-js/loc.js
+++ b/src/language-js/loc.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { isNonEmptyArray } = require("../common/util");
+const { isNonEmptyArray } = require("../common/util.js");
 
 /**
  * @typedef {import("./types/estree").Node} Node
diff --git a/src/language-js/needs-parens.js b/src/language-js/needs-parens.js
index 466d05dcf..3a8bcd57c 100644
--- a/src/language-js/needs-parens.js
+++ b/src/language-js/needs-parens.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const getLast = require("../utils/get-last");
+const getLast = require("../utils/get-last.js");
 const {
   getFunctionParameters,
   getLeftSidePathName,
@@ -14,7 +14,7 @@ const {
   isCallExpression,
   isMemberExpression,
   isObjectProperty,
-} = require("./utils");
+} = require("./utils.js");
 
 function needsParens(path, options) {
   const parent = path.getParentNode();
diff --git a/src/language-js/options.js b/src/language-js/options.js
index 775d7c7ee..e04541f06 100644
--- a/src/language-js/options.js
+++ b/src/language-js/options.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const commonOptions = require("../common/common-options");
+const commonOptions = require("../common/common-options.js");
 
 const CATEGORY_JAVASCRIPT = "JavaScript";
 
diff --git a/src/language-js/parse/angular.js b/src/language-js/parse/angular.js
index 8a7420f77..b35a9d192 100644
--- a/src/language-js/parse/angular.js
+++ b/src/language-js/parse/angular.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { locStart, locEnd } = require("../loc");
+const { locStart, locEnd } = require("../loc.js");
 
 function createParser(_parse) {
   const parse = (text, parsers, options) => {
diff --git a/src/language-js/parse/babel.js b/src/language-js/parse/babel.js
index 6a2e0db53..b2221f39a 100644
--- a/src/language-js/parse/babel.js
+++ b/src/language-js/parse/babel.js
@@ -1,14 +1,14 @@
 "use strict";
 
-const tryCombinations = require("../../utils/try-combinations");
+const tryCombinations = require("../../utils/try-combinations.js");
 const {
   getNextNonSpaceNonCommentCharacterIndexWithStartIndex,
   getShebang,
-} = require("../../common/util");
-const createParser = require("./utils/create-parser");
-const createBabelParseError = require("./utils/create-babel-parse-error");
-const postprocess = require("./postprocess");
-const jsonParsers = require("./json");
+} = require("../../common/util.js");
+const createParser = require("./utils/create-parser.js");
+const createBabelParseError = require("./utils/create-babel-parse-error.js");
+const postprocess = require("./postprocess.js");
+const jsonParsers = require("./json.js");
 
 const parseOptions = {
   sourceType: "module",
diff --git a/src/language-js/parse/espree.js b/src/language-js/parse/espree.js
index 55827f5ce..550eeff1a 100644
--- a/src/language-js/parse/espree.js
+++ b/src/language-js/parse/espree.js
@@ -1,10 +1,10 @@
 "use strict";
 
-const createError = require("../../common/parser-create-error");
-const tryCombinations = require("../../utils/try-combinations");
-const createParser = require("./utils/create-parser");
-const replaceHashbang = require("./utils/replace-hashbang");
-const postprocess = require("./postprocess");
+const createError = require("../../common/parser-create-error.js");
+const tryCombinations = require("../../utils/try-combinations.js");
+const createParser = require("./utils/create-parser.js");
+const replaceHashbang = require("./utils/replace-hashbang.js");
+const postprocess = require("./postprocess.js");
 
 /** @type {import("espree").Options} */
 const parseOptions = {
diff --git a/src/language-js/parse/flow.js b/src/language-js/parse/flow.js
index b9854af15..c880d3dd0 100644
--- a/src/language-js/parse/flow.js
+++ b/src/language-js/parse/flow.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const createError = require("../../common/parser-create-error");
-const createParser = require("./utils/create-parser");
-const replaceHashbang = require("./utils/replace-hashbang");
-const postprocess = require("./postprocess");
+const createError = require("../../common/parser-create-error.js");
+const createParser = require("./utils/create-parser.js");
+const replaceHashbang = require("./utils/replace-hashbang.js");
+const postprocess = require("./postprocess.js");
 
 // https://github.com/facebook/flow/tree/master/packages/flow-parser#options
 const parseOptions = {
diff --git a/src/language-js/parse/json.js b/src/language-js/parse/json.js
index b7fe221e0..02885cfd2 100644
--- a/src/language-js/parse/json.js
+++ b/src/language-js/parse/json.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const { isNonEmptyArray } = require("../../common/util");
-const createError = require("../../common/parser-create-error");
-const createParser = require("./utils/create-parser");
-const createBabelParseError = require("./utils/create-babel-parse-error");
+const { isNonEmptyArray } = require("../../common/util.js");
+const createError = require("../../common/parser-create-error.js");
+const createParser = require("./utils/create-parser.js");
+const createBabelParseError = require("./utils/create-babel-parse-error.js");
 
 function createJsonParse(options = {}) {
   const { allowComments = true } = options;
diff --git a/src/language-js/parse/meriyah.js b/src/language-js/parse/meriyah.js
index 8e8672e0f..dd20373b8 100644
--- a/src/language-js/parse/meriyah.js
+++ b/src/language-js/parse/meriyah.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const createError = require("../../common/parser-create-error");
-const tryCombinations = require("../../utils/try-combinations");
-const createParser = require("./utils/create-parser");
-const postprocess = require("./postprocess");
+const createError = require("../../common/parser-create-error.js");
+const tryCombinations = require("../../utils/try-combinations.js");
+const createParser = require("./utils/create-parser.js");
+const postprocess = require("./postprocess.js");
 
 // https://github.com/meriyah/meriyah/blob/4676f60b6c149d7082bde2c9147f9ae2359c8075/src/parser.ts#L185
 const parseOptions = {
diff --git a/src/language-js/parse/parsers.js b/src/language-js/parse/parsers.js
index 0f0374f20..0d06399d1 100644
--- a/src/language-js/parse/parsers.js
+++ b/src/language-js/parse/parsers.js
@@ -3,66 +3,66 @@
 module.exports = {
   // JS - Babel
   get babel() {
-    return require("./babel").parsers.babel;
+    return require("./babel.js").parsers.babel;
   },
   get "babel-flow"() {
-    return require("./babel").parsers["babel-flow"];
+    return require("./babel.js").parsers["babel-flow"];
   },
   get "babel-ts"() {
-    return require("./babel").parsers["babel-ts"];
+    return require("./babel.js").parsers["babel-ts"];
   },
   get json() {
-    return require("./babel").parsers.json;
+    return require("./babel.js").parsers.json;
   },
   get json5() {
-    return require("./babel").parsers.json5;
+    return require("./babel.js").parsers.json5;
   },
   get "json-stringify"() {
-    return require("./babel").parsers["json-stringify"];
+    return require("./babel.js").parsers["json-stringify"];
   },
   get __js_expression() {
-    return require("./babel").parsers.__js_expression;
+    return require("./babel.js").parsers.__js_expression;
   },
   get __vue_expression() {
-    return require("./babel").parsers.__vue_expression;
+    return require("./babel.js").parsers.__vue_expression;
   },
   get __vue_event_binding() {
-    return require("./babel").parsers.__vue_event_binding;
+    return require("./babel.js").parsers.__vue_event_binding;
   },
   // JS - Flow
   get flow() {
-    return require("./flow").parsers.flow;
+    return require("./flow.js").parsers.flow;
   },
   // JS - TypeScript
   get typescript() {
-    return require("./typescript").parsers.typescript;
+    return require("./typescript.js").parsers.typescript;
   },
   // JS - Angular Action
   get __ng_action() {
-    return require("./angular").parsers.__ng_action;
+    return require("./angular.js").parsers.__ng_action;
   },
   // JS - Angular Binding
   get __ng_binding() {
-    return require("./angular").parsers.__ng_binding;
+    return require("./angular.js").parsers.__ng_binding;
   },
   // JS - Angular Interpolation
   get __ng_interpolation() {
-    return require("./angular").parsers.__ng_interpolation;
+    return require("./angular.js").parsers.__ng_interpolation;
   },
   // JS - Angular Directive
   get __ng_directive() {
-    return require("./angular").parsers.__ng_directive;
+    return require("./angular.js").parsers.__ng_directive;
   },
   // JS - espree
   get espree() {
-    return require("./espree").parsers.espree;
+    return require("./espree.js").parsers.espree;
   },
   // JS - meriyah
   get meriyah() {
-    return require("./meriyah").parsers.meriyah;
+    return require("./meriyah.js").parsers.meriyah;
   },
   // JS - Babel Estree
   get __babel_estree() {
-    return require("./babel").parsers.__babel_estree;
+    return require("./babel.js").parsers.__babel_estree;
   },
 };
diff --git a/src/language-js/parse/postprocess.js b/src/language-js/parse/postprocess.js
index 047fc3ece..18e1afd63 100644
--- a/src/language-js/parse/postprocess.js
+++ b/src/language-js/parse/postprocess.js
@@ -3,10 +3,10 @@
 const {
   getLast,
   getNextNonSpaceNonCommentCharacter,
-} = require("../../common/util");
-const createError = require("../../common/parser-create-error");
-const { locStart, locEnd } = require("../loc");
-const { isTypeCastComment } = require("../comments");
+} = require("../../common/util.js");
+const createError = require("../../common/parser-create-error.js");
+const { locStart, locEnd } = require("../loc.js");
+const { isTypeCastComment } = require("../comments.js");
 
 function postprocess(ast, options) {
   // Invalid decorators are removed since `@typescript-eslint/typescript-estree` v4
diff --git a/src/language-js/parse/typescript.js b/src/language-js/parse/typescript.js
index d573dade9..cbdc09ad1 100644
--- a/src/language-js/parse/typescript.js
+++ b/src/language-js/parse/typescript.js
@@ -1,10 +1,10 @@
 "use strict";
 
-const createError = require("../../common/parser-create-error");
-const tryCombinations = require("../../utils/try-combinations");
-const createParser = require("./utils/create-parser");
-const replaceHashbang = require("./utils/replace-hashbang");
-const postprocess = require("./postprocess");
+const createError = require("../../common/parser-create-error.js");
+const tryCombinations = require("../../utils/try-combinations.js");
+const createParser = require("./utils/create-parser.js");
+const replaceHashbang = require("./utils/replace-hashbang.js");
+const postprocess = require("./postprocess.js");
 
 /** @type {import("@typescript-eslint/typescript-estree").TSESTreeOptions} */
 const parseOptions = {
diff --git a/src/language-js/parse/utils/create-babel-parse-error.js b/src/language-js/parse/utils/create-babel-parse-error.js
index 10ae2b1d1..a1e2ffa2a 100644
--- a/src/language-js/parse/utils/create-babel-parse-error.js
+++ b/src/language-js/parse/utils/create-babel-parse-error.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const createError = require("../../../common/parser-create-error");
+const createError = require("../../../common/parser-create-error.js");
 
 function createBabelParseError(error) {
   // babel error prints (l:c) with cols that are zero indexed
diff --git a/src/language-js/parse/utils/create-parser.js b/src/language-js/parse/utils/create-parser.js
index 34d803287..be4f3d49e 100644
--- a/src/language-js/parse/utils/create-parser.js
+++ b/src/language-js/parse/utils/create-parser.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const { hasPragma } = require("../../pragma");
-const { locStart, locEnd } = require("../../loc");
+const { hasPragma } = require("../../pragma.js");
+const { locStart, locEnd } = require("../../loc.js");
 
 function createParser(options) {
   options = typeof options === "function" ? { parse: options } : options;
diff --git a/src/language-js/pragma.js b/src/language-js/pragma.js
index 3299ad3cb..c243fe5c6 100644
--- a/src/language-js/pragma.js
+++ b/src/language-js/pragma.js
@@ -1,8 +1,8 @@
 "use strict";
 
 const { parseWithComments, strip, extract, print } = require("jest-docblock");
-const { getShebang } = require("../common/util");
-const { normalizeEndOfLine } = require("../common/end-of-line");
+const { getShebang } = require("../common/util.js");
+const { normalizeEndOfLine } = require("../common/end-of-line.js");
 
 function parseDocBlock(text) {
   const shebang = getShebang(text);
diff --git a/src/language-js/print/angular.js b/src/language-js/print/angular.js
index 8f30adf77..0f6821ae7 100644
--- a/src/language-js/print/angular.js
+++ b/src/language-js/print/angular.js
@@ -2,9 +2,9 @@
 
 const {
   builders: { join, line, group },
-} = require("../../document");
-const { hasNode, hasComment, getComments } = require("../utils");
-const { printBinaryishExpression } = require("./binaryish");
+} = require("../../document/index.js");
+const { hasNode, hasComment, getComments } = require("../utils.js");
+const { printBinaryishExpression } = require("./binaryish.js");
 
 /** @typedef {import("../../common/ast-path")} AstPath */
 
diff --git a/src/language-js/print/array.js b/src/language-js/print/array.js
index 5d07163b5..632ea7b67 100644
--- a/src/language-js/print/array.js
+++ b/src/language-js/print/array.js
@@ -1,10 +1,10 @@
 "use strict";
 
-const { printDanglingComments } = require("../../main/comments");
+const { printDanglingComments } = require("../../main/comments.js");
 const {
   builders: { line, softline, hardline, group, indent, ifBreak, fill },
-} = require("../../document");
-const { getLast, hasNewline } = require("../../common/util");
+} = require("../../document/index.js");
+const { getLast, hasNewline } = require("../../common/util.js");
 const {
   shouldPrintComma,
   hasComment,
@@ -12,10 +12,10 @@ const {
   isNextLineEmpty,
   isNumericLiteral,
   isSignedNumericLiteral,
-} = require("../utils");
-const { locStart } = require("../loc");
+} = require("../utils.js");
+const { locStart } = require("../loc.js");
 
-const { printOptionalToken, printTypeAnnotation } = require("./misc");
+const { printOptionalToken, printTypeAnnotation } = require("./misc.js");
 
 /** @typedef {import("../../document").Doc} Doc */
 
diff --git a/src/language-js/print/assignment.js b/src/language-js/print/assignment.js
index 2a2adfe76..db343a9eb 100644
--- a/src/language-js/print/assignment.js
+++ b/src/language-js/print/assignment.js
@@ -1,10 +1,10 @@
 "use strict";
 
-const { isNonEmptyArray, getStringWidth } = require("../../common/util");
+const { isNonEmptyArray, getStringWidth } = require("../../common/util.js");
 const {
   builders: { line, group, indent, indentIfBreak },
   utils: { cleanDoc, willBreak },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   hasLeadingOwnLineComment,
   isBinaryish,
@@ -18,9 +18,9 @@ const {
   hasComment,
   isSignedNumericLiteral,
   isObjectProperty,
-} = require("../utils");
-const { shouldInlineLogicalExpression } = require("./binaryish");
-const { printCallExpression } = require("./call-expression");
+} = require("../utils.js");
+const { shouldInlineLogicalExpression } = require("./binaryish.js");
+const { printCallExpression } = require("./call-expression.js");
 
 function printAssignment(
   path,
diff --git a/src/language-js/print/binaryish.js b/src/language-js/print/binaryish.js
index 402bf2e6f..15caa6ca3 100644
--- a/src/language-js/print/binaryish.js
+++ b/src/language-js/print/binaryish.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const { printComments } = require("../../main/comments");
-const { getLast } = require("../../common/util");
+const { printComments } = require("../../main/comments.js");
+const { getLast } = require("../../common/util.js");
 const {
   builders: {
     join,
@@ -14,7 +14,7 @@ const {
     indentIfBreak,
   },
   utils: { cleanDoc, getDocParts, isConcat },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   hasLeadingOwnLineComment,
   isBinaryish,
@@ -25,7 +25,7 @@ const {
   isCallExpression,
   isMemberExpression,
   isObjectProperty,
-} = require("../utils");
+} = require("../utils.js");
 
 /** @typedef {import("../../document").Doc} Doc */
 
diff --git a/src/language-js/print/block.js b/src/language-js/print/block.js
index 28ff518a8..af3445cd2 100644
--- a/src/language-js/print/block.js
+++ b/src/language-js/print/block.js
@@ -1,14 +1,18 @@
 "use strict";
 
-const { printDanglingComments } = require("../../main/comments");
-const { isNonEmptyArray } = require("../../common/util");
+const { printDanglingComments } = require("../../main/comments.js");
+const { isNonEmptyArray } = require("../../common/util.js");
 const {
   builders: { hardline, indent },
-} = require("../../document");
-const { hasComment, CommentCheckFlags, isNextLineEmpty } = require("../utils");
-const { printHardlineAfterHeritage } = require("./class");
+} = require("../../document/index.js");
+const {
+  hasComment,
+  CommentCheckFlags,
+  isNextLineEmpty,
+} = require("../utils.js");
+const { printHardlineAfterHeritage } = require("./class.js");
 
-const { printBody } = require("./statement");
+const { printBody } = require("./statement.js");
 
 /** @typedef {import("../../document").Doc} Doc */
 
diff --git a/src/language-js/print/call-arguments.js b/src/language-js/print/call-arguments.js
index c415e836e..e99cdf8d2 100644
--- a/src/language-js/print/call-arguments.js
+++ b/src/language-js/print/call-arguments.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const { printDanglingComments } = require("../../main/comments");
-const { getLast, getPenultimate } = require("../../common/util");
+const { printDanglingComments } = require("../../main/comments.js");
+const { getLast, getPenultimate } = require("../../common/util.js");
 const {
   getFunctionParameters,
   hasComment,
@@ -16,7 +16,7 @@ const {
   isCallExpression,
   isStringLiteral,
   isObjectProperty,
-} = require("../utils");
+} = require("../utils.js");
 
 const {
   builders: {
@@ -30,10 +30,10 @@ const {
     breakParent,
   },
   utils: { willBreak },
-} = require("../../document");
+} = require("../../document/index.js");
 
-const { ArgExpansionBailout } = require("../../common/errors");
-const { isConciselyPrintedArray } = require("./array");
+const { ArgExpansionBailout } = require("../../common/errors.js");
+const { isConciselyPrintedArray } = require("./array.js");
 
 function printCallArguments(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/print/call-expression.js b/src/language-js/print/call-expression.js
index 81962434e..071291ee8 100644
--- a/src/language-js/print/call-expression.js
+++ b/src/language-js/print/call-expression.js
@@ -2,8 +2,8 @@
 
 const {
   builders: { join, group },
-} = require("../../document");
-const pathNeedsParens = require("../needs-parens");
+} = require("../../document/index.js");
+const pathNeedsParens = require("../needs-parens.js");
 const {
   getCallArguments,
   hasFlowAnnotationComment,
@@ -13,10 +13,13 @@ const {
   isTemplateOnItsOwnLine,
   isTestCall,
   iterateCallArgumentsPath,
-} = require("../utils");
-const printMemberChain = require("./member-chain");
-const printCallArguments = require("./call-arguments");
-const { printOptionalToken, printFunctionTypeParameters } = require("./misc");
+} = require("../utils.js");
+const printMemberChain = require("./member-chain.js");
+const printCallArguments = require("./call-arguments.js");
+const {
+  printOptionalToken,
+  printFunctionTypeParameters,
+} = require("./misc.js");
 
 function printCallExpression(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/print/class.js b/src/language-js/print/class.js
index 61bb7ea05..017f9a36d 100644
--- a/src/language-js/print/class.js
+++ b/src/language-js/print/class.js
@@ -1,17 +1,23 @@
 "use strict";
 
-const { isNonEmptyArray, createGroupIdMapper } = require("../../common/util");
-const { printComments, printDanglingComments } = require("../../main/comments");
+const {
+  isNonEmptyArray,
+  createGroupIdMapper,
+} = require("../../common/util.js");
+const {
+  printComments,
+  printDanglingComments,
+} = require("../../main/comments.js");
 const {
   builders: { join, line, hardline, softline, group, indent, ifBreak },
-} = require("../../document");
-const { hasComment, CommentCheckFlags } = require("../utils");
-const { getTypeParametersGroupId } = require("./type-parameters");
-const { printMethod } = require("./function");
-const { printOptionalToken, printTypeAnnotation } = require("./misc");
-const { printPropertyKey } = require("./property");
-const { printAssignment } = require("./assignment");
-const { printClassMemberDecorators } = require("./decorators");
+} = require("../../document/index.js");
+const { hasComment, CommentCheckFlags } = require("../utils.js");
+const { getTypeParametersGroupId } = require("./type-parameters.js");
+const { printMethod } = require("./function.js");
+const { printOptionalToken, printTypeAnnotation } = require("./misc.js");
+const { printPropertyKey } = require("./property.js");
+const { printAssignment } = require("./assignment.js");
+const { printClassMemberDecorators } = require("./decorators.js");
 
 function printClass(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/print/comment.js b/src/language-js/print/comment.js
index b972d8fa4..a6b350833 100644
--- a/src/language-js/print/comment.js
+++ b/src/language-js/print/comment.js
@@ -1,13 +1,13 @@
 "use strict";
 
-const { hasNewline } = require("../../common/util");
+const { hasNewline } = require("../../common/util.js");
 const {
   builders: { join, hardline },
   utils: { replaceNewlinesWithLiterallines },
-} = require("../../document");
+} = require("../../document/index.js");
 
-const { isLineComment, isBlockComment } = require("../utils");
-const { locStart, locEnd } = require("../loc");
+const { isLineComment, isBlockComment } = require("../utils.js");
+const { locStart, locEnd } = require("../loc.js");
 
 function printComment(commentPath, options) {
   const comment = commentPath.getValue();
diff --git a/src/language-js/print/decorators.js b/src/language-js/print/decorators.js
index db2ecd1d2..393cc38b3 100644
--- a/src/language-js/print/decorators.js
+++ b/src/language-js/print/decorators.js
@@ -1,11 +1,11 @@
 "use strict";
 
-const { isNonEmptyArray, hasNewline } = require("../../common/util");
+const { isNonEmptyArray, hasNewline } = require("../../common/util.js");
 const {
   builders: { line, hardline, join, breakParent, group },
-} = require("../../document");
-const { locStart, locEnd } = require("../loc");
-const { getParentExportDeclaration } = require("../utils");
+} = require("../../document/index.js");
+const { locStart, locEnd } = require("../loc.js");
+const { getParentExportDeclaration } = require("../utils.js");
 
 function printClassMemberDecorators(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/print/flow.js b/src/language-js/print/flow.js
index 092665c34..090d9adf4 100644
--- a/src/language-js/print/flow.js
+++ b/src/language-js/print/flow.js
@@ -3,20 +3,20 @@
 /** @typedef {import("../../document").Doc} Doc */
 
 const assert = require("assert");
-const { printDanglingComments } = require("../../main/comments");
-const { printString, printNumber } = require("../../common/util");
+const { printDanglingComments } = require("../../main/comments.js");
+const { printString, printNumber } = require("../../common/util.js");
 const {
   builders: { hardline, softline, group, indent },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   getParentExportDeclaration,
   isFunctionNotation,
   isGetterOrSetter,
   rawText,
   shouldPrintComma,
-} = require("../utils");
-const { locStart, locEnd } = require("../loc");
-const { printClass } = require("./class");
+} = require("../utils.js");
+const { locStart, locEnd } = require("../loc.js");
+const { printClass } = require("./class.js");
 const {
   printOpaqueType,
   printTypeAlias,
@@ -25,24 +25,24 @@ const {
   printFunctionType,
   printTupleType,
   printIndexedAccessType,
-} = require("./type-annotation");
-const { printInterface } = require("./interface");
+} = require("./type-annotation.js");
+const { printInterface } = require("./interface.js");
 const {
   printTypeParameter,
   printTypeParameters,
-} = require("./type-parameters");
+} = require("./type-parameters.js");
 const {
   printExportDeclaration,
   printExportAllDeclaration,
-} = require("./module");
-const { printArrayItems } = require("./array");
-const { printObject } = require("./object");
-const { printPropertyKey } = require("./property");
+} = require("./module.js");
+const { printArrayItems } = require("./array.js");
+const { printObject } = require("./object.js");
+const { printPropertyKey } = require("./property.js");
 const {
   printOptionalToken,
   printTypeAnnotation,
   printRestSpread,
-} = require("./misc");
+} = require("./misc.js");
 
 function printFlow(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/print/function-parameters.js b/src/language-js/print/function-parameters.js
index cd5ad46b9..1d9a29a16 100644
--- a/src/language-js/print/function-parameters.js
+++ b/src/language-js/print/function-parameters.js
@@ -1,11 +1,11 @@
 "use strict";
 
-const { getNextNonSpaceNonCommentCharacter } = require("../../common/util");
-const { printDanglingComments } = require("../../main/comments");
+const { getNextNonSpaceNonCommentCharacter } = require("../../common/util.js");
+const { printDanglingComments } = require("../../main/comments.js");
 const {
   builders: { line, hardline, softline, group, indent, ifBreak },
   utils: { removeLines, willBreak },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   getFunctionParameters,
   iterateFunctionParametersPath,
@@ -18,10 +18,10 @@ const {
   shouldPrintComma,
   hasComment,
   isNextLineEmpty,
-} = require("../utils");
-const { locEnd } = require("../loc");
-const { ArgExpansionBailout } = require("../../common/errors");
-const { printFunctionTypeParameters } = require("./misc");
+} = require("../utils.js");
+const { locEnd } = require("../loc.js");
+const { ArgExpansionBailout } = require("../../common/errors.js");
+const { printFunctionTypeParameters } = require("./misc.js");
 
 function printFunctionParameters(
   path,
diff --git a/src/language-js/print/function.js b/src/language-js/print/function.js
index 78109891d..8ba8618a1 100644
--- a/src/language-js/print/function.js
+++ b/src/language-js/print/function.js
@@ -6,11 +6,11 @@ const assert = require("assert");
 const {
   printDanglingComments,
   printCommentsSeparately,
-} = require("../../main/comments");
-const getLast = require("../../utils/get-last");
+} = require("../../main/comments.js");
+const getLast = require("../../utils/get-last.js");
 const {
   getNextNonSpaceNonCommentCharacterIndex,
-} = require("../../common/util");
+} = require("../../common/util.js");
 const {
   builders: {
     line,
@@ -23,8 +23,8 @@ const {
     indentIfBreak,
   },
   utils: { removeLines, willBreak },
-} = require("../../document");
-const { ArgExpansionBailout } = require("../../common/errors");
+} = require("../../document/index.js");
+const { ArgExpansionBailout } = require("../../common/errors.js");
 const {
   getFunctionParameters,
   hasLeadingOwnLineComment,
@@ -43,14 +43,14 @@ const {
   getCallArguments,
   hasNakedLeftSide,
   getLeftSide,
-} = require("../utils");
-const { locEnd } = require("../loc");
+} = require("../utils.js");
+const { locEnd } = require("../loc.js");
 const {
   printFunctionParameters,
   shouldGroupFunctionParameters,
-} = require("./function-parameters");
-const { printPropertyKey } = require("./property");
-const { printFunctionTypeParameters } = require("./misc");
+} = require("./function-parameters.js");
+const { printPropertyKey } = require("./property.js");
+const { printFunctionTypeParameters } = require("./misc.js");
 
 function printFunction(path, print, options, args) {
   const node = path.getValue();
diff --git a/src/language-js/print/html-binding.js b/src/language-js/print/html-binding.js
index 61a5d8cee..6932b74bd 100644
--- a/src/language-js/print/html-binding.js
+++ b/src/language-js/print/html-binding.js
@@ -2,7 +2,7 @@
 
 const {
   builders: { join, line, group, softline, indent },
-} = require("../../document");
+} = require("../../document/index.js");
 
 function printHtmlBinding(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/print/interface.js b/src/language-js/print/interface.js
index d8bfcd0d4..9b422e4b7 100644
--- a/src/language-js/print/interface.js
+++ b/src/language-js/print/interface.js
@@ -1,12 +1,12 @@
 "use strict";
 
-const { isNonEmptyArray } = require("../../common/util");
+const { isNonEmptyArray } = require("../../common/util.js");
 const {
   builders: { join, line, group, indent, ifBreak },
-} = require("../../document");
-const { hasComment, identity, CommentCheckFlags } = require("../utils");
-const { getTypeParametersGroupId } = require("./type-parameters");
-const { printTypeScriptModifiers } = require("./misc");
+} = require("../../document/index.js");
+const { hasComment, identity, CommentCheckFlags } = require("../utils.js");
+const { getTypeParametersGroupId } = require("./type-parameters.js");
+const { printTypeScriptModifiers } = require("./misc.js");
 
 function printInterface(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/print/jsx.js b/src/language-js/print/jsx.js
index 3b1f6ae38..4c4df813c 100644
--- a/src/language-js/print/jsx.js
+++ b/src/language-js/print/jsx.js
@@ -1,6 +1,9 @@
 "use strict";
 
-const { printComments, printDanglingComments } = require("../../main/comments");
+const {
+  printComments,
+  printDanglingComments,
+} = require("../../main/comments.js");
 const {
   builders: {
     line,
@@ -15,9 +18,9 @@ const {
     join,
   },
   utils: { willBreak },
-} = require("../../document");
+} = require("../../document/index.js");
 
-const { getLast, getPreferredQuote } = require("../../common/util");
+const { getLast, getPreferredQuote } = require("../../common/util.js");
 const {
   isJsxNode,
   rawText,
@@ -28,9 +31,9 @@ const {
   hasComment,
   CommentCheckFlags,
   hasNodeIgnoreComment,
-} = require("../utils");
-const pathNeedsParens = require("../needs-parens");
-const { willPrintOwnComments } = require("../comments");
+} = require("../utils.js");
+const pathNeedsParens = require("../needs-parens.js");
+const { willPrintOwnComments } = require("../comments.js");
 
 const isEmptyStringOrAnyLine = (doc) =>
   doc === "" || doc === line || doc === hardline || doc === softline;
diff --git a/src/language-js/print/literal.js b/src/language-js/print/literal.js
index 6eae023d8..15cb41afa 100644
--- a/src/language-js/print/literal.js
+++ b/src/language-js/print/literal.js
@@ -1,5 +1,5 @@
 "use strict";
-const { printString, printNumber } = require("../../common/util");
+const { printString, printNumber } = require("../../common/util.js");
 
 function printLiteral(path, options /*, print*/) {
   const node = path.getNode();
diff --git a/src/language-js/print/member-chain.js b/src/language-js/print/member-chain.js
index fefca1fdd..3a3d761bc 100644
--- a/src/language-js/print/member-chain.js
+++ b/src/language-js/print/member-chain.js
@@ -1,12 +1,12 @@
 "use strict";
 
-const { printComments } = require("../../main/comments");
+const { printComments } = require("../../main/comments.js");
 const {
   getLast,
   isNextLineEmptyAfterIndex,
   getNextNonSpaceNonCommentCharacterIndex,
-} = require("../../common/util");
-const pathNeedsParens = require("../needs-parens");
+} = require("../../common/util.js");
+const pathNeedsParens = require("../needs-parens.js");
 const {
   isCallExpression,
   isMemberExpression,
@@ -18,8 +18,8 @@ const {
   hasComment,
   CommentCheckFlags,
   isNextLineEmpty,
-} = require("../utils");
-const { locEnd } = require("../loc");
+} = require("../utils.js");
+const { locEnd } = require("../loc.js");
 
 const {
   builders: {
@@ -32,14 +32,14 @@ const {
     label,
   },
   utils: { willBreak },
-} = require("../../document");
-const printCallArguments = require("./call-arguments");
-const { printMemberLookup } = require("./member");
+} = require("../../document/index.js");
+const printCallArguments = require("./call-arguments.js");
+const { printMemberLookup } = require("./member.js");
 const {
   printOptionalToken,
   printFunctionTypeParameters,
   printBindExpressionCallee,
-} = require("./misc");
+} = require("./misc.js");
 
 // We detect calls on member expressions specially to format a
 // common pattern better. The pattern we are looking for is this:
diff --git a/src/language-js/print/member.js b/src/language-js/print/member.js
index af0a620dd..b594f332c 100644
--- a/src/language-js/print/member.js
+++ b/src/language-js/print/member.js
@@ -2,13 +2,13 @@
 
 const {
   builders: { softline, group, indent, label },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   isNumericLiteral,
   isMemberExpression,
   isCallExpression,
-} = require("../utils");
-const { printOptionalToken } = require("./misc");
+} = require("../utils.js");
+const { printOptionalToken } = require("./misc.js");
 
 function printMemberExpression(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/print/misc.js b/src/language-js/print/misc.js
index c4afd15a1..b40d076a2 100644
--- a/src/language-js/print/misc.js
+++ b/src/language-js/print/misc.js
@@ -1,10 +1,10 @@
 "use strict";
 
-const { isNonEmptyArray } = require("../../common/util");
+const { isNonEmptyArray } = require("../../common/util.js");
 const {
   builders: { indent, join, line },
-} = require("../../document");
-const { isFlowAnnotationComment } = require("../utils");
+} = require("../../document/index.js");
+const { isFlowAnnotationComment } = require("../utils.js");
 
 function printOptionalToken(path) {
   const node = path.getValue();
diff --git a/src/language-js/print/module.js b/src/language-js/print/module.js
index a32b495ed..aee1f85e5 100644
--- a/src/language-js/print/module.js
+++ b/src/language-js/print/module.js
@@ -1,22 +1,22 @@
 "use strict";
 
-const { isNonEmptyArray } = require("../../common/util");
+const { isNonEmptyArray } = require("../../common/util.js");
 const {
   builders: { softline, group, indent, join, line, ifBreak, hardline },
-} = require("../../document");
-const { printDanglingComments } = require("../../main/comments");
+} = require("../../document/index.js");
+const { printDanglingComments } = require("../../main/comments.js");
 
 const {
   hasComment,
   CommentCheckFlags,
   shouldPrintComma,
   needsHardlineAfterDanglingComment,
-} = require("../utils");
-const { locStart, hasSameLoc } = require("../loc");
+} = require("../utils.js");
+const { locStart, hasSameLoc } = require("../loc.js");
 const {
   hasDecoratorsBeforeExport,
   printDecoratorsBeforeExport,
-} = require("./decorators");
+} = require("./decorators.js");
 
 /**
  * @typedef {import("../../document").Doc} Doc
diff --git a/src/language-js/print/object.js b/src/language-js/print/object.js
index 00afcfc6e..bce115b51 100644
--- a/src/language-js/print/object.js
+++ b/src/language-js/print/object.js
@@ -1,28 +1,28 @@
 "use strict";
 
-const { printDanglingComments } = require("../../main/comments");
+const { printDanglingComments } = require("../../main/comments.js");
 const {
   builders: { line, softline, group, indent, ifBreak, hardline },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   getLast,
   hasNewlineInRange,
   hasNewline,
   isNonEmptyArray,
-} = require("../../common/util");
+} = require("../../common/util.js");
 const {
   shouldPrintComma,
   hasComment,
   getComments,
   CommentCheckFlags,
   isNextLineEmpty,
-} = require("../utils");
-const { locStart, locEnd } = require("../loc");
+} = require("../utils.js");
+const { locStart, locEnd } = require("../loc.js");
 
-const { printOptionalToken, printTypeAnnotation } = require("./misc");
-const { shouldHugFunctionParameters } = require("./function-parameters");
-const { shouldHugType } = require("./type-annotation");
-const { printHardlineAfterHeritage } = require("./class");
+const { printOptionalToken, printTypeAnnotation } = require("./misc.js");
+const { shouldHugFunctionParameters } = require("./function-parameters.js");
+const { shouldHugType } = require("./type-annotation.js");
+const { printHardlineAfterHeritage } = require("./class.js");
 
 /** @typedef {import("../../document").Doc} Doc */
 
diff --git a/src/language-js/print/property.js b/src/language-js/print/property.js
index b598c2c93..ca85bd9d5 100644
--- a/src/language-js/print/property.js
+++ b/src/language-js/print/property.js
@@ -1,15 +1,15 @@
 "use strict";
 
-const { printComments } = require("../../main/comments");
-const { printString, printNumber } = require("../../common/util");
+const { printComments } = require("../../main/comments.js");
+const { printString, printNumber } = require("../../common/util.js");
 const {
   isNumericLiteral,
   isSimpleNumber,
   isStringLiteral,
   isStringPropSafeToUnquote,
   rawText,
-} = require("../utils");
-const { printAssignment } = require("./assignment");
+} = require("../utils.js");
+const { printAssignment } = require("./assignment.js");
 
 const needsQuoteProps = new WeakMap();
 
diff --git a/src/language-js/print/statement.js b/src/language-js/print/statement.js
index 76365ec21..9dc33a742 100644
--- a/src/language-js/print/statement.js
+++ b/src/language-js/print/statement.js
@@ -2,8 +2,8 @@
 
 const {
   builders: { hardline },
-} = require("../../document");
-const pathNeedsParens = require("../needs-parens");
+} = require("../../document/index.js");
+const pathNeedsParens = require("../needs-parens.js");
 const {
   getLeftSidePathName,
   hasNakedLeftSide,
@@ -12,8 +12,8 @@ const {
   hasComment,
   CommentCheckFlags,
   isNextLineEmpty,
-} = require("../utils");
-const { shouldPrintParamsWithoutParens } = require("./function");
+} = require("../utils.js");
+const { shouldPrintParamsWithoutParens } = require("./function.js");
 
 /**
  * @typedef {import("../../document").Doc} Doc
diff --git a/src/language-js/print/template-literal.js b/src/language-js/print/template-literal.js
index cfa61da53..02ca550ff 100644
--- a/src/language-js/print/template-literal.js
+++ b/src/language-js/print/template-literal.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const getLast = require("../../utils/get-last");
-const { getStringWidth, getIndentSize } = require("../../common/util");
+const getLast = require("../../utils/get-last.js");
+const { getStringWidth, getIndentSize } = require("../../common/util.js");
 const {
   builders: {
     join,
@@ -15,14 +15,14 @@ const {
   },
   printer: { printDocToString },
   utils: { mapDoc },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   isBinaryish,
   isJestEachTemplateLiteral,
   isSimpleTemplateLiteral,
   hasComment,
   isMemberExpression,
-} = require("../utils");
+} = require("../utils.js");
 
 function printTemplateLiteral(path, print, options) {
   const node = path.getValue();
diff --git a/src/language-js/print/ternary.js b/src/language-js/print/ternary.js
index 9f1c8aa2d..60551ddf7 100644
--- a/src/language-js/print/ternary.js
+++ b/src/language-js/print/ternary.js
@@ -1,14 +1,14 @@
 "use strict";
 
-const { hasNewlineInRange } = require("../../common/util");
+const { hasNewlineInRange } = require("../../common/util.js");
 const {
   isJsxNode,
   isBlockComment,
   getComments,
   isCallExpression,
   isMemberExpression,
-} = require("../utils");
-const { locStart, locEnd } = require("../loc");
+} = require("../utils.js");
+const { locStart, locEnd } = require("../loc.js");
 const {
   builders: {
     line,
@@ -20,7 +20,7 @@ const {
     dedent,
     breakParent,
   },
-} = require("../../document");
+} = require("../../document/index.js");
 
 /**
  * @typedef {import("../../document").Doc} Doc
diff --git a/src/language-js/print/type-annotation.js b/src/language-js/print/type-annotation.js
index f37f518ba..7772e9099 100644
--- a/src/language-js/print/type-annotation.js
+++ b/src/language-js/print/type-annotation.js
@@ -1,25 +1,28 @@
 "use strict";
 
-const { printComments, printDanglingComments } = require("../../main/comments");
-const { getLast } = require("../../common/util");
+const {
+  printComments,
+  printDanglingComments,
+} = require("../../main/comments.js");
+const { getLast } = require("../../common/util.js");
 const {
   builders: { group, join, line, softline, indent, align, ifBreak },
-} = require("../../document");
-const pathNeedsParens = require("../needs-parens");
-const { locStart } = require("../loc");
+} = require("../../document/index.js");
+const pathNeedsParens = require("../needs-parens.js");
+const { locStart } = require("../loc.js");
 const {
   isSimpleType,
   isObjectType,
   hasLeadingOwnLineComment,
   isObjectTypePropertyAFunction,
   shouldPrintComma,
-} = require("../utils");
-const { printAssignment } = require("./assignment");
+} = require("../utils.js");
+const { printAssignment } = require("./assignment.js");
 const {
   printFunctionParameters,
   shouldGroupFunctionParameters,
-} = require("./function-parameters");
-const { printArrayItems } = require("./array");
+} = require("./function-parameters.js");
+const { printArrayItems } = require("./array.js");
 
 function shouldHugType(node) {
   if (isSimpleType(node) || isObjectType(node)) {
diff --git a/src/language-js/print/type-parameters.js b/src/language-js/print/type-parameters.js
index f9aeaa334..3f9f81c26 100644
--- a/src/language-js/print/type-parameters.js
+++ b/src/language-js/print/type-parameters.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const { printDanglingComments } = require("../../main/comments");
+const { printDanglingComments } = require("../../main/comments.js");
 const {
   builders: { join, line, hardline, softline, group, indent, ifBreak },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   isTestCall,
   hasComment,
@@ -11,9 +11,9 @@ const {
   isTSXFile,
   shouldPrintComma,
   getFunctionParameters,
-} = require("../utils");
-const { createGroupIdMapper } = require("../../common/util");
-const { shouldHugType } = require("./type-annotation");
+} = require("../utils.js");
+const { createGroupIdMapper } = require("../../common/util.js");
+const { shouldHugType } = require("./type-annotation.js");
 
 const getTypeParametersGroupId = createGroupIdMapper("typeParameters");
 
diff --git a/src/language-js/print/typescript.js b/src/language-js/print/typescript.js
index 3e7f1b88e..f66856f99 100644
--- a/src/language-js/print/typescript.js
+++ b/src/language-js/print/typescript.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const { printDanglingComments } = require("../../main/comments");
-const { hasNewlineInRange } = require("../../common/util");
+const { printDanglingComments } = require("../../main/comments.js");
+const { hasNewlineInRange } = require("../../common/util.js");
 const {
   builders: {
     join,
@@ -13,34 +13,34 @@ const {
     conditionalGroup,
     ifBreak,
   },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   isLiteral,
   getTypeScriptMappedTypeModifier,
   shouldPrintComma,
   isCallExpression,
   isMemberExpression,
-} = require("../utils");
-const { locStart, locEnd } = require("../loc");
+} = require("../utils.js");
+const { locStart, locEnd } = require("../loc.js");
 
-const { printOptionalToken, printTypeScriptModifiers } = require("./misc");
-const { printTernary } = require("./ternary");
+const { printOptionalToken, printTypeScriptModifiers } = require("./misc.js");
+const { printTernary } = require("./ternary.js");
 const {
   printFunctionParameters,
   shouldGroupFunctionParameters,
-} = require("./function-parameters");
-const { printTemplateLiteral } = require("./template-literal");
-const { printArrayItems } = require("./array");
-const { printObject } = require("./object");
-const { printClassProperty, printClassMethod } = require("./class");
+} = require("./function-parameters.js");
+const { printTemplateLiteral } = require("./template-literal.js");
+const { printArrayItems } = require("./array.js");
+const { printObject } = require("./object.js");
+const { printClassProperty, printClassMethod } = require("./class.js");
 const {
   printTypeParameter,
   printTypeParameters,
-} = require("./type-parameters");
-const { printPropertyKey } = require("./property");
-const { printFunction, printMethodInternal } = require("./function");
-const { printInterface } = require("./interface");
-const { printBlock } = require("./block");
+} = require("./type-parameters.js");
+const { printPropertyKey } = require("./property.js");
+const { printFunction, printMethodInternal } = require("./function.js");
+const { printInterface } = require("./interface.js");
+const { printBlock } = require("./block.js");
 const {
   printTypeAlias,
   printIntersectionType,
@@ -48,7 +48,7 @@ const {
   printFunctionType,
   printTupleType,
   printIndexedAccessType,
-} = require("./type-annotation");
+} = require("./type-annotation.js");
 
 function printTypescript(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/printer-estree-json.js b/src/language-js/printer-estree-json.js
index 5fa591948..d3bc27c51 100644
--- a/src/language-js/printer-estree-json.js
+++ b/src/language-js/printer-estree-json.js
@@ -2,8 +2,8 @@
 
 const {
   builders: { hardline, indent, join },
-} = require("../document");
-const preprocess = require("./print-preprocess");
+} = require("../document/index.js");
+const preprocess = require("./print-preprocess.js");
 
 function genericPrint(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-js/printer-estree.js b/src/language-js/printer-estree.js
index 75f881a87..8b15a5bea 100644
--- a/src/language-js/printer-estree.js
+++ b/src/language-js/printer-estree.js
@@ -3,18 +3,18 @@
 /** @typedef {import("../document").Doc} Doc */
 
 // TODO(azz): anything that imports from main shouldn't be in a `language-*` dir.
-const { printDanglingComments } = require("../main/comments");
-const { hasNewline } = require("../common/util");
+const { printDanglingComments } = require("../main/comments.js");
+const { hasNewline } = require("../common/util.js");
 const {
   builders: { join, line, hardline, softline, group, indent },
   utils: { replaceNewlinesWithLiterallines },
-} = require("../document");
-const embed = require("./embed");
-const clean = require("./clean");
-const { insertPragma } = require("./pragma");
-const handleComments = require("./comments");
-const pathNeedsParens = require("./needs-parens");
-const preprocess = require("./print-preprocess");
+} = require("../document/index.js");
+const embed = require("./embed.js");
+const clean = require("./clean.js");
+const { insertPragma } = require("./pragma.js");
+const handleComments = require("./comments.js");
+const pathNeedsParens = require("./needs-parens.js");
+const preprocess = require("./print-preprocess.js");
 const {
   hasFlowShorthandAnnotationComment,
   hasComment,
@@ -28,59 +28,59 @@ const {
   hasIgnoreComment,
   isCallExpression,
   isMemberExpression,
-} = require("./utils");
-const { locStart, locEnd } = require("./loc");
+} = require("./utils.js");
+const { locStart, locEnd } = require("./loc.js");
 
 const {
   printHtmlBinding,
   isVueEventBindingExpression,
-} = require("./print/html-binding");
-const { printAngular } = require("./print/angular");
-const { printJsx, hasJsxIgnoreComment } = require("./print/jsx");
-const { printFlow } = require("./print/flow");
-const { printTypescript } = require("./print/typescript");
+} = require("./print/html-binding.js");
+const { printAngular } = require("./print/angular.js");
+const { printJsx, hasJsxIgnoreComment } = require("./print/jsx.js");
+const { printFlow } = require("./print/flow.js");
+const { printTypescript } = require("./print/typescript.js");
 const {
   printOptionalToken,
   printBindExpressionCallee,
   printTypeAnnotation,
   adjustClause,
   printRestSpread,
-} = require("./print/misc");
+} = require("./print/misc.js");
 const {
   printImportDeclaration,
   printExportDeclaration,
   printExportAllDeclaration,
   printModuleSpecifier,
-} = require("./print/module");
-const { printTernary } = require("./print/ternary");
-const { printTemplateLiteral } = require("./print/template-literal");
-const { printArray } = require("./print/array");
-const { printObject } = require("./print/object");
+} = require("./print/module.js");
+const { printTernary } = require("./print/ternary.js");
+const { printTemplateLiteral } = require("./print/template-literal.js");
+const { printArray } = require("./print/array.js");
+const { printObject } = require("./print/object.js");
 const {
   printClass,
   printClassMethod,
   printClassProperty,
-} = require("./print/class");
-const { printProperty } = require("./print/property");
+} = require("./print/class.js");
+const { printProperty } = require("./print/property.js");
 const {
   printFunction,
   printArrowFunction,
   printMethod,
   printReturnStatement,
   printThrowStatement,
-} = require("./print/function");
-const { printCallExpression } = require("./print/call-expression");
+} = require("./print/function.js");
+const { printCallExpression } = require("./print/call-expression.js");
 const {
   printVariableDeclarator,
   printAssignmentExpression,
-} = require("./print/assignment");
-const { printBinaryishExpression } = require("./print/binaryish");
-const { printSwitchCaseConsequent } = require("./print/statement");
-const { printMemberExpression } = require("./print/member");
-const { printBlock, printBlockBody } = require("./print/block");
-const { printComment } = require("./print/comment");
-const { printLiteral } = require("./print/literal");
-const { printDecorators } = require("./print/decorators");
+} = require("./print/assignment.js");
+const { printBinaryishExpression } = require("./print/binaryish.js");
+const { printSwitchCaseConsequent } = require("./print/statement.js");
+const { printMemberExpression } = require("./print/member.js");
+const { printBlock, printBlockBody } = require("./print/block.js");
+const { printComment } = require("./print/comment.js");
+const { printLiteral } = require("./print/literal.js");
+const { printDecorators } = require("./print/decorators.js");
 
 function genericPrint(path, options, print, args) {
   const printed = printPathNoParens(path, options, print, args);
diff --git a/src/language-js/utils.js b/src/language-js/utils.js
index 9aefc897b..7dd9ab44e 100644
--- a/src/language-js/utils.js
+++ b/src/language-js/utils.js
@@ -7,8 +7,8 @@ const {
   skipWhitespace,
   isNonEmptyArray,
   isNextLineEmptyAfterIndex,
-} = require("../common/util");
-const { locStart, locEnd, hasSameLocStart } = require("./loc");
+} = require("../common/util.js");
+const { locStart, locEnd, hasSameLocStart } = require("./loc.js");
 
 /**
  * @typedef {import("./types/estree").Node} Node
diff --git a/src/language-markdown/clean.js b/src/language-markdown/clean.js
index 869eef9dd..60c3c7caa 100644
--- a/src/language-markdown/clean.js
+++ b/src/language-markdown/clean.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const { isFrontMatterNode } = require("../common/util");
-const { startWithPragma } = require("./pragma");
+const { isFrontMatterNode } = require("../common/util.js");
+const { startWithPragma } = require("./pragma.js");
 
 const ignoredProperties = new Set([
   "position",
diff --git a/src/language-markdown/embed.js b/src/language-markdown/embed.js
index aa3641c76..006f97905 100644
--- a/src/language-markdown/embed.js
+++ b/src/language-markdown/embed.js
@@ -3,13 +3,13 @@
 const {
   inferParserByLanguage,
   getMaxContinuousCount,
-} = require("../common/util");
+} = require("../common/util.js");
 const {
   builders: { hardline, markAsRoot },
   utils: { replaceNewlinesWithLiterallines },
-} = require("../document");
-const printFrontMatter = require("../utils/front-matter/print");
-const { getFencedCodeBlockValue } = require("./utils");
+} = require("../document/index.js");
+const printFrontMatter = require("../utils/front-matter/print.js");
+const { getFencedCodeBlockValue } = require("./utils.js");
 
 function embed(path, print, textToDoc, options) {
   const node = path.getValue();
diff --git a/src/language-markdown/index.js b/src/language-markdown/index.js
index 695e97a1c..7d195073c 100644
--- a/src/language-markdown/index.js
+++ b/src/language-markdown/index.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const createLanguage = require("../utils/create-language");
-const printer = require("./printer-markdown");
-const options = require("./options");
-const parsers = require("./parsers");
+const createLanguage = require("../utils/create-language.js");
+const printer = require("./printer-markdown.js");
+const options = require("./options.js");
+const parsers = require("./parsers.js");
 
 const languages = [
   createLanguage(require("linguist-languages/data/Markdown.json"), (data) => ({
diff --git a/src/language-markdown/options.js b/src/language-markdown/options.js
index 87e20f73c..edb5fd4c4 100644
--- a/src/language-markdown/options.js
+++ b/src/language-markdown/options.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const commonOptions = require("../common/common-options");
+const commonOptions = require("../common/common-options.js");
 
 // format based on https://github.com/prettier/prettier/blob/main/src/main/core-options.js
 module.exports = {
diff --git a/src/language-markdown/parser-markdown.js b/src/language-markdown/parser-markdown.js
index 60b19e52f..773c7dbc5 100644
--- a/src/language-markdown/parser-markdown.js
+++ b/src/language-markdown/parser-markdown.js
@@ -4,11 +4,11 @@ const remarkParse = require("remark-parse");
 const unified = require("unified");
 const remarkMath = require("remark-math");
 const footnotes = require("remark-footnotes");
-const parseFrontMatter = require("../utils/front-matter/parse");
-const pragma = require("./pragma");
-const { locStart, locEnd } = require("./loc");
-const { mapAst, INLINE_NODE_WRAPPER_TYPES } = require("./utils");
-const mdx = require("./mdx");
+const parseFrontMatter = require("../utils/front-matter/parse.js");
+const pragma = require("./pragma.js");
+const { locStart, locEnd } = require("./loc.js");
+const { mapAst, INLINE_NODE_WRAPPER_TYPES } = require("./utils.js");
+const mdx = require("./mdx.js");
 
 /**
  * based on [MDAST](https://github.com/syntax-tree/mdast) with following modifications:
diff --git a/src/language-markdown/parsers.js b/src/language-markdown/parsers.js
index 3a1e5da12..333e56cd6 100644
--- a/src/language-markdown/parsers.js
+++ b/src/language-markdown/parsers.js
@@ -3,12 +3,12 @@
 module.exports = {
   /* istanbul ignore next */
   get remark() {
-    return require("./parser-markdown").parsers.remark;
+    return require("./parser-markdown.js").parsers.remark;
   },
   get markdown() {
-    return require("./parser-markdown").parsers.remark;
+    return require("./parser-markdown.js").parsers.remark;
   },
   get mdx() {
-    return require("./parser-markdown").parsers.mdx;
+    return require("./parser-markdown.js").parsers.mdx;
   },
 };
diff --git a/src/language-markdown/pragma.js b/src/language-markdown/pragma.js
index ef7e98042..862e0f285 100644
--- a/src/language-markdown/pragma.js
+++ b/src/language-markdown/pragma.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const parseFrontMatter = require("../utils/front-matter/parse");
+const parseFrontMatter = require("../utils/front-matter/parse.js");
 
 const pragmas = ["format", "prettier"];
 
diff --git a/src/language-markdown/print-preprocess.js b/src/language-markdown/print-preprocess.js
index b816fa882..eb177afcb 100644
--- a/src/language-markdown/print-preprocess.js
+++ b/src/language-markdown/print-preprocess.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const getLast = require("../utils/get-last");
-const { getOrderedListItemInfo, mapAst, splitText } = require("./utils");
+const getLast = require("../utils/get-last.js");
+const { getOrderedListItemInfo, mapAst, splitText } = require("./utils.js");
 
 // 0x0 ~ 0x10ffff
 const isSingleCharRegex = /^.$/su;
diff --git a/src/language-markdown/printer-markdown.js b/src/language-markdown/printer-markdown.js
index 29f938d41..9553276cb 100644
--- a/src/language-markdown/printer-markdown.js
+++ b/src/language-markdown/printer-markdown.js
@@ -6,7 +6,7 @@ const {
   getMaxContinuousCount,
   getStringWidth,
   isNonEmptyArray,
-} = require("../common/util");
+} = require("../common/util.js");
 const {
   builders: {
     breakParent,
@@ -25,12 +25,12 @@ const {
   },
   utils: { normalizeDoc, replaceEndOfLineWith },
   printer: { printDocToString },
-} = require("../document");
-const embed = require("./embed");
-const { insertPragma } = require("./pragma");
-const { locStart, locEnd } = require("./loc");
-const preprocess = require("./print-preprocess");
-const clean = require("./clean");
+} = require("../document/index.js");
+const embed = require("./embed.js");
+const { insertPragma } = require("./pragma.js");
+const { locStart, locEnd } = require("./loc.js");
+const preprocess = require("./print-preprocess.js");
+const clean = require("./clean.js");
 const {
   getFencedCodeBlockValue,
   hasGitDiffFriendlyOrderedList,
@@ -39,7 +39,7 @@ const {
   INLINE_NODE_TYPES,
   INLINE_NODE_WRAPPER_TYPES,
   isAutolink,
-} = require("./utils");
+} = require("./utils.js");
 
 /**
  * @typedef {import("../document").Doc} Doc
diff --git a/src/language-markdown/utils.js b/src/language-markdown/utils.js
index 4b7d2608e..b3f4a9090 100644
--- a/src/language-markdown/utils.js
+++ b/src/language-markdown/utils.js
@@ -1,12 +1,12 @@
 "use strict";
 
-const { getLast } = require("../common/util");
-const { locStart, locEnd } = require("./loc");
+const { getLast } = require("../common/util.js");
+const { locStart, locEnd } = require("./loc.js");
 const {
   cjkPattern,
   kPattern,
   punctuationPattern,
-} = require("./constants.evaluate");
+} = require("./constants.evaluate.js");
 
 const INLINE_NODE_TYPES = [
   "liquidNode",
diff --git a/src/language-yaml/index.js b/src/language-yaml/index.js
index adb68c414..f406e04e5 100644
--- a/src/language-yaml/index.js
+++ b/src/language-yaml/index.js
@@ -1,9 +1,9 @@
 "use strict";
 
-const createLanguage = require("../utils/create-language");
-const printer = require("./printer-yaml");
-const options = require("./options");
-const parsers = require("./parsers");
+const createLanguage = require("../utils/create-language.js");
+const printer = require("./printer-yaml.js");
+const options = require("./options.js");
+const parsers = require("./parsers.js");
 
 const languages = [
   createLanguage(require("linguist-languages/data/YAML.json"), (data) => ({
diff --git a/src/language-yaml/options.js b/src/language-yaml/options.js
index a25c8cde7..3911f0603 100644
--- a/src/language-yaml/options.js
+++ b/src/language-yaml/options.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const commonOptions = require("../common/common-options");
+const commonOptions = require("../common/common-options.js");
 
 // format based on https://github.com/prettier/prettier/blob/main/src/main/core-options.js
 module.exports = {
diff --git a/src/language-yaml/parser-yaml.js b/src/language-yaml/parser-yaml.js
index 58a0d40e1..d89995a34 100644
--- a/src/language-yaml/parser-yaml.js
+++ b/src/language-yaml/parser-yaml.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const createError = require("../common/parser-create-error");
-const { hasPragma } = require("./pragma");
-const { locStart, locEnd } = require("./loc");
+const createError = require("../common/parser-create-error.js");
+const { hasPragma } = require("./pragma.js");
+const { locStart, locEnd } = require("./loc.js");
 
 function parse(text) {
   const { parse } = require("yaml-unist-parser");
diff --git a/src/language-yaml/parsers.js b/src/language-yaml/parsers.js
index ed1595f1b..de543e7d9 100644
--- a/src/language-yaml/parsers.js
+++ b/src/language-yaml/parsers.js
@@ -2,6 +2,6 @@
 
 module.exports = {
   get yaml() {
-    return require("./parser-yaml").parsers.yaml;
+    return require("./parser-yaml.js").parsers.yaml;
   },
 };
diff --git a/src/language-yaml/print-preprocess.js b/src/language-yaml/print-preprocess.js
index 79912a1c0..f1744ecad 100644
--- a/src/language-yaml/print-preprocess.js
+++ b/src/language-yaml/print-preprocess.js
@@ -1,5 +1,5 @@
 "use strict";
-const { defineShortcut, mapNode } = require("./utils");
+const { defineShortcut, mapNode } = require("./utils.js");
 
 function preprocess(ast) {
   return mapNode(ast, defineShortcuts);
diff --git a/src/language-yaml/print/block.js b/src/language-yaml/print/block.js
index 9e8e8e615..afbc82112 100644
--- a/src/language-yaml/print/block.js
+++ b/src/language-yaml/print/block.js
@@ -14,15 +14,15 @@ const {
     markAsRoot,
   },
   utils: { getDocParts },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   getAncestorCount,
   getBlockValueLineContents,
   hasIndicatorComment,
   isLastDescendantNode,
   isNode,
-} = require("../utils");
-const { alignWithSpaces } = require("./misc");
+} = require("../utils.js");
+const { alignWithSpaces } = require("./misc.js");
 
 function printBlock(path, print, options) {
   const node = path.getValue();
diff --git a/src/language-yaml/print/flow-mapping-sequence.js b/src/language-yaml/print/flow-mapping-sequence.js
index 7b7e234ef..3f3077d93 100644
--- a/src/language-yaml/print/flow-mapping-sequence.js
+++ b/src/language-yaml/print/flow-mapping-sequence.js
@@ -2,9 +2,9 @@
 
 const {
   builders: { ifBreak, line, softline, hardline, join },
-} = require("../../document");
-const { isEmptyNode, getLast, hasEndComments } = require("../utils");
-const { printNextEmptyLine, alignWithSpaces } = require("./misc");
+} = require("../../document/index.js");
+const { isEmptyNode, getLast, hasEndComments } = require("../utils.js");
+const { printNextEmptyLine, alignWithSpaces } = require("./misc.js");
 
 function printFlowMapping(path, print, options) {
   const node = path.getValue();
diff --git a/src/language-yaml/print/mapping-item.js b/src/language-yaml/print/mapping-item.js
index d669fc350..98c7dcb23 100644
--- a/src/language-yaml/print/mapping-item.js
+++ b/src/language-yaml/print/mapping-item.js
@@ -4,7 +4,7 @@
 
 const {
   builders: { conditionalGroup, group, hardline, ifBreak, join, line },
-} = require("../../document");
+} = require("../../document/index.js");
 const {
   hasLeadingComments,
   hasMiddleComments,
@@ -13,8 +13,8 @@ const {
   isNode,
   isEmptyNode,
   isInlineNode,
-} = require("../utils");
-const { alignWithSpaces } = require("./misc");
+} = require("../utils.js");
+const { alignWithSpaces } = require("./misc.js");
 
 function printMappingItem(node, parentNode, path, print, options) {
   const { key, value } = node;
diff --git a/src/language-yaml/print/misc.js b/src/language-yaml/print/misc.js
index 25d87fe08..117a551ed 100644
--- a/src/language-yaml/print/misc.js
+++ b/src/language-yaml/print/misc.js
@@ -2,9 +2,9 @@
 
 const {
   builders: { softline, align },
-} = require("../../document");
+} = require("../../document/index.js");
 
-const { hasEndComments, isNextLineEmpty, isNode } = require("../utils");
+const { hasEndComments, isNextLineEmpty, isNode } = require("../utils.js");
 
 const printedEmptyLineCache = new WeakMap();
 function printNextEmptyLine(path, originalText) {
diff --git a/src/language-yaml/printer-yaml.js b/src/language-yaml/printer-yaml.js
index b358a449f..d9ec4dc33 100644
--- a/src/language-yaml/printer-yaml.js
+++ b/src/language-yaml/printer-yaml.js
@@ -14,11 +14,11 @@ const {
     literalline,
   },
   utils: { getDocParts, replaceEndOfLineWith },
-} = require("../document");
-const { isPreviousLineEmpty } = require("../common/util");
-const { insertPragma, isPragma } = require("./pragma");
-const { locStart } = require("./loc");
-const embed = require("./embed");
+} = require("../document/index.js");
+const { isPreviousLineEmpty } = require("../common/util.js");
+const { insertPragma, isPragma } = require("./pragma.js");
+const { locStart } = require("./loc.js");
+const embed = require("./embed.js");
 const {
   getFlowScalarLineContents,
   getLastDescendantNode,
@@ -30,19 +30,19 @@ const {
   isLastDescendantNode,
   isNode,
   isInlineNode,
-} = require("./utils");
-const preprocess = require("./print-preprocess");
+} = require("./utils.js");
+const preprocess = require("./print-preprocess.js");
 const {
   alignWithSpaces,
   printNextEmptyLine,
   shouldPrintEndComments,
-} = require("./print/misc");
+} = require("./print/misc.js");
 const {
   printFlowMapping,
   printFlowSequence,
-} = require("./print/flow-mapping-sequence");
-const printMappingItem = require("./print/mapping-item");
-const printBlock = require("./print/block");
+} = require("./print/flow-mapping-sequence.js");
+const printMappingItem = require("./print/mapping-item.js");
+const printBlock = require("./print/block.js");
 
 function genericPrint(path, options, print) {
   const node = path.getValue();
diff --git a/src/language-yaml/utils.js b/src/language-yaml/utils.js
index 99514f34c..904ffc8ce 100644
--- a/src/language-yaml/utils.js
+++ b/src/language-yaml/utils.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const { getLast, isNonEmptyArray } = require("../common/util");
+const { getLast, isNonEmptyArray } = require("../common/util.js");
 
 function getAncestorCount(path, filter) {
   let counter = 0;
diff --git a/src/languages.js b/src/languages.js
index 7ea31f024..46c216cea 100644
--- a/src/languages.js
+++ b/src/languages.js
@@ -3,17 +3,17 @@
 // We need to list the parsers and getters so we can load them only when necessary.
 module.exports = [
   // JS
-  require("./language-js"),
+  require("./language-js/index.js"),
   // CSS
-  require("./language-css"),
+  require("./language-css/index.js"),
   // Handlebars
-  require("./language-handlebars"),
+  require("./language-handlebars/index.js"),
   // GraphQL
-  require("./language-graphql"),
+  require("./language-graphql/index.js"),
   // Markdown
-  require("./language-markdown"),
+  require("./language-markdown/index.js"),
   // HTML
-  require("./language-html"),
+  require("./language-html/index.js"),
   // YAML
-  require("./language-yaml"),
+  require("./language-yaml/index.js"),
 ];
diff --git a/src/main/ast-to-doc.js b/src/main/ast-to-doc.js
index 0122a9614..46d6b8d5d 100644
--- a/src/main/ast-to-doc.js
+++ b/src/main/ast-to-doc.js
@@ -1,12 +1,12 @@
 "use strict";
 
-const AstPath = require("../common/ast-path");
+const AstPath = require("../common/ast-path.js");
 const {
   builders: { hardline, addAlignmentToDoc },
   utils: { propagateBreaks },
-} = require("../document");
-const { printComments } = require("./comments");
-const multiparser = require("./multiparser");
+} = require("../document/index.js");
+const { printComments } = require("./comments.js");
+const multiparser = require("./multiparser.js");
 
 /**
  * Takes an abstract syntax tree (AST) and recursively converts it to a
diff --git a/src/main/comments.js b/src/main/comments.js
index 3547ad2de..2562cf260 100644
--- a/src/main/comments.js
+++ b/src/main/comments.js
@@ -4,7 +4,7 @@ const assert = require("assert");
 
 const {
   builders: { line, hardline, breakParent, indent, lineSuffix, join, cursor },
-} = require("../document");
+} = require("../document/index.js");
 
 const {
   hasNewline,
@@ -14,7 +14,7 @@ const {
   addLeadingComment,
   addDanglingComment,
   addTrailingComment,
-} = require("../common/util");
+} = require("../common/util.js");
 
 const childNodesCache = new WeakMap();
 function getSortedChildNodes(node, options, resultArray) {
diff --git a/src/main/core.js b/src/main/core.js
index 059f95da4..48035d355 100644
--- a/src/main/core.js
+++ b/src/main/core.js
@@ -5,20 +5,20 @@ const diff = require("diff");
 const {
   printer: { printDocToString },
   debug: { printDocToDebug },
-} = require("../document");
-const { getAlignmentSize } = require("../common/util");
+} = require("../document/index.js");
+const { getAlignmentSize } = require("../common/util.js");
 const {
   guessEndOfLine,
   convertEndOfLineToChars,
   countEndOfLineChars,
   normalizeEndOfLine,
-} = require("../common/end-of-line");
-const normalizeOptions = require("./options").normalize;
-const massageAST = require("./massage-ast");
-const comments = require("./comments");
-const parser = require("./parser");
-const printAstToDoc = require("./ast-to-doc");
-const rangeUtil = require("./range-util");
+} = require("../common/end-of-line.js");
+const normalizeOptions = require("./options.js").normalize;
+const massageAST = require("./massage-ast.js");
+const comments = require("./comments.js");
+const parser = require("./parser.js");
+const printAstToDoc = require("./ast-to-doc.js");
+const rangeUtil = require("./range-util.js");
 
 const BOM = "\uFEFF";
 
diff --git a/src/main/multiparser.js b/src/main/multiparser.js
index 017b73447..3af8b24f7 100644
--- a/src/main/multiparser.js
+++ b/src/main/multiparser.js
@@ -2,9 +2,9 @@
 
 const {
   utils: { stripTrailingHardline },
-} = require("../document");
-const { normalize } = require("./options");
-const comments = require("./comments");
+} = require("../document/index.js");
+const { normalize } = require("./options.js");
+const comments = require("./comments.js");
 
 function printSubtree(path, print, options, printAstToDoc) {
   if (options.printer.embed && options.embeddedLanguageFormatting === "auto") {
@@ -42,7 +42,7 @@ function textToDoc(
     { passThrough: true }
   );
 
-  const result = require("./parser").parse(text, nextOptions);
+  const result = require("./parser.js").parse(text, nextOptions);
   const { ast } = result;
   text = result.text;
 
diff --git a/src/main/options-normalizer.js b/src/main/options-normalizer.js
index 43c805062..3240a083c 100644
--- a/src/main/options-normalizer.js
+++ b/src/main/options-normalizer.js
@@ -3,7 +3,7 @@
 const vnopts = require("vnopts");
 const leven = require("leven");
 const chalk = require("chalk");
-const getLast = require("../utils/get-last");
+const getLast = require("../utils/get-last.js");
 
 const cliDescriptor = {
   key: (key) => (key.length === 1 ? `-${key}` : `--${key}`),
diff --git a/src/main/options.js b/src/main/options.js
index 17d25d801..e6fe2be29 100644
--- a/src/main/options.js
+++ b/src/main/options.js
@@ -3,10 +3,10 @@
 const fs = require("fs");
 const path = require("path");
 const readlines = require("n-readlines");
-const { UndefinedParserError } = require("../common/errors");
-const { getSupportInfo } = require("../main/support");
-const normalizer = require("./options-normalizer");
-const { resolveParser } = require("./parser");
+const { UndefinedParserError } = require("../common/errors.js");
+const { getSupportInfo } = require("../main/support.js");
+const normalizer = require("./options-normalizer.js");
+const { resolveParser } = require("./parser.js");
 
 const hiddenDefaults = {
   astFormat: "estree",
diff --git a/src/main/parser.js b/src/main/parser.js
index 5f221d5ba..ea2ff5b2c 100644
--- a/src/main/parser.js
+++ b/src/main/parser.js
@@ -1,8 +1,8 @@
 "use strict";
 
 const path = require("path");
-const { ConfigError } = require("../common/errors");
-const jsLoc = require("../language-js/loc");
+const { ConfigError } = require("../common/errors.js");
+const jsLoc = require("../language-js/loc.js");
 
 const { locStart, locEnd } = jsLoc;
 
diff --git a/src/main/range-util.js b/src/main/range-util.js
index 473a56a07..0570a0bfb 100644
--- a/src/main/range-util.js
+++ b/src/main/range-util.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const assert = require("assert");
-const comments = require("./comments");
+const comments = require("./comments.js");
 
 const isJsonParser = ({ parser }) =>
   parser === "json" || parser === "json5" || parser === "json-stringify";
diff --git a/src/main/support.js b/src/main/support.js
index 8e0a85016..6bf3ad8d8 100644
--- a/src/main/support.js
+++ b/src/main/support.js
@@ -5,9 +5,9 @@ const semver = {
   lt: require("semver/functions/lt"),
   gte: require("semver/functions/gte"),
 };
-const arrayify = require("../utils/arrayify");
+const arrayify = require("../utils/arrayify.js");
 const currentVersion = require("../../package.json").version;
-const coreOptions = require("./core-options").options;
+const coreOptions = require("./core-options.js").options;
 
 /**
  * Strings in `plugins` and `pluginSearchDirs` are handled by a wrapped version
diff --git a/src/standalone.js b/src/standalone.js
index baf6836dc..21b190849 100644
--- a/src/standalone.js
+++ b/src/standalone.js
@@ -2,11 +2,11 @@
 
 const { version } = require("../package.json");
 
-const core = require("./main/core");
-const { getSupportInfo } = require("./main/support");
-const sharedUtil = require("./common/util-shared");
-const languages = require("./languages");
-const doc = require("./document");
+const core = require("./main/core.js");
+const { getSupportInfo } = require("./main/support.js");
+const sharedUtil = require("./common/util-shared.js");
+const languages = require("./languages.js");
+const doc = require("./document/index.js");
 
 function withPlugins(
   fn,
diff --git a/src/utils/front-matter/print.js b/src/utils/front-matter/print.js
index d28089735..5de6ee784 100644
--- a/src/utils/front-matter/print.js
+++ b/src/utils/front-matter/print.js
@@ -2,7 +2,7 @@
 
 const {
   builders: { hardline, markAsRoot },
-} = require("../../document");
+} = require("../../document/index.js");
 
 function print(node, textToDoc) {
   if (node.lang === "yaml") {
diff --git a/standalone.js b/standalone.js
index 6b3dd4c62..c202611da 100644
--- a/standalone.js
+++ b/standalone.js
@@ -1,3 +1,3 @@
 "use strict";
 
-module.exports = require("./src/standalone");
+module.exports = require("./src/standalone.js");
diff --git a/tests/config/format-test.js b/tests/config/format-test.js
index 06b5204c2..41d5091fc 100644
--- a/tests/config/format-test.js
+++ b/tests/config/format-test.js
@@ -7,11 +7,11 @@ const path = require("path");
 const prettier = !TEST_STANDALONE
   ? require("prettier-local")
   : require("prettier-standalone");
-const checkParsers = require("./utils/check-parsers");
-const createSnapshot = require("./utils/create-snapshot");
-const visualizeEndOfLine = require("./utils/visualize-end-of-line");
-const consistentEndOfLine = require("./utils/consistent-end-of-line");
-const stringifyOptionsForTitle = require("./utils/stringify-options-for-title");
+const checkParsers = require("./utils/check-parsers.js");
+const createSnapshot = require("./utils/create-snapshot.js");
+const visualizeEndOfLine = require("./utils/visualize-end-of-line.js");
+const consistentEndOfLine = require("./utils/consistent-end-of-line.js");
+const stringifyOptionsForTitle = require("./utils/stringify-options-for-title.js");
 
 const { FULL_TEST } = process.env;
 const BOM = "\uFEFF";
diff --git a/tests/config/prettier-plugins/prettier-plugin-uppercase-rocks/index.js b/tests/config/prettier-plugins/prettier-plugin-uppercase-rocks/index.js
index 534d67a8c..9e1c50c60 100644
--- a/tests/config/prettier-plugins/prettier-plugin-uppercase-rocks/index.js
+++ b/tests/config/prettier-plugins/prettier-plugin-uppercase-rocks/index.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const createPlugin = require("../../utils/create-plugin");
+const createPlugin = require("../../utils/create-plugin.js");
 
 module.exports = createPlugin({
   name: "uppercase-rocks",
diff --git a/tests/config/setup.js b/tests/config/setup.js
index 7ae0b4cd7..d32f0bf79 100644
--- a/tests/config/setup.js
+++ b/tests/config/setup.js
@@ -2,6 +2,6 @@
 
 Object.defineProperty(global, "run_spec", {
   get() {
-    return require("./format-test");
+    return require("./format-test.js");
   },
 });
diff --git a/tests/config/utils/create-snapshot.js b/tests/config/utils/create-snapshot.js
index 7a61acd2b..a889a2730 100644
--- a/tests/config/utils/create-snapshot.js
+++ b/tests/config/utils/create-snapshot.js
@@ -1,8 +1,8 @@
 "use strict";
 
 const raw = require("jest-snapshot-serializer-raw").wrap;
-const visualizeRange = require("./visualize-range");
-const visualizeEndOfLine = require("./visualize-end-of-line");
+const visualizeRange = require("./visualize-range.js");
+const visualizeEndOfLine = require("./visualize-end-of-line.js");
 
 const SEPARATOR_WIDTH = 80;
 function printSeparator(description = "") {
diff --git a/tests/format/markdown/broken-plugins/jsfmt.spec.js b/tests/format/markdown/broken-plugins/jsfmt.spec.js
index e161ca4df..f798cb1ff 100644
--- a/tests/format/markdown/broken-plugins/jsfmt.spec.js
+++ b/tests/format/markdown/broken-plugins/jsfmt.spec.js
@@ -1,5 +1,5 @@
 const plugins = [
-  require("../../../config/prettier-plugins/prettier-plugin-missing-comments/"),
+  require("../../../config/prettier-plugins/prettier-plugin-missing-comments/index.js"),
 ];
 
 run_spec(__dirname, ["markdown"], { plugins });
diff --git a/tests/format/misc/errors/broken-plugin/jsfmt.spec.js b/tests/format/misc/errors/broken-plugin/jsfmt.spec.js
index 9b0f14811..826d73513 100644
--- a/tests/format/misc/errors/broken-plugin/jsfmt.spec.js
+++ b/tests/format/misc/errors/broken-plugin/jsfmt.spec.js
@@ -1,5 +1,5 @@
 const plugins = [
-  require("../../../../config/prettier-plugins/prettier-plugin-missing-comments/"),
+  require("../../../../config/prettier-plugins/prettier-plugin-missing-comments/index.js"),
 ];
 
 run_spec({ dirname: __dirname, snippets: ["text"] }, ["missing-comments"], {
diff --git a/tests/format/vue/broken-plugins/jsfmt.spec.js b/tests/format/vue/broken-plugins/jsfmt.spec.js
index e84d59ad7..decac9184 100644
--- a/tests/format/vue/broken-plugins/jsfmt.spec.js
+++ b/tests/format/vue/broken-plugins/jsfmt.spec.js
@@ -1,5 +1,5 @@
 const plugins = [
-  require("../../../config/prettier-plugins/prettier-plugin-missing-comments/"),
+  require("../../../config/prettier-plugins/prettier-plugin-missing-comments/index.js"),
 ];
 
 run_spec(__dirname, ["vue"], { plugins });
diff --git a/tests/format/vue/with-plugins/jsfmt.spec.js b/tests/format/vue/with-plugins/jsfmt.spec.js
index 8a992cf5f..8300a31c1 100644
--- a/tests/format/vue/with-plugins/jsfmt.spec.js
+++ b/tests/format/vue/with-plugins/jsfmt.spec.js
@@ -1,5 +1,5 @@
 const plugins = [
-  require("../../../config/prettier-plugins/prettier-plugin-uppercase-rocks/"),
+  require("../../../config/prettier-plugins/prettier-plugin-uppercase-rocks/index.js"),
 ];
 
 run_spec(__dirname, ["vue"], { plugins });
diff --git a/tests/integration/__tests__/arg-parsing.js b/tests/integration/__tests__/arg-parsing.js
index 70a2dab38..0fafcc983 100644
--- a/tests/integration/__tests__/arg-parsing.js
+++ b/tests/integration/__tests__/arg-parsing.js
@@ -1,7 +1,7 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
-expect.addSnapshotSerializer(require("../path-serializer"));
+const runPrettier = require("../runPrettier.js");
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("boolean flags do not swallow the next argument", () => {
   runPrettier("cli/arg-parsing", [
diff --git a/tests/integration/__tests__/bundle.js b/tests/integration/__tests__/bundle.js
index 2138bb81f..831a26cd2 100644
--- a/tests/integration/__tests__/bundle.js
+++ b/tests/integration/__tests__/bundle.js
@@ -2,9 +2,10 @@
 
 const path = require("path");
 const globby = require("globby");
-const { projectRoot } = require("../env");
-const coreOptions = require("../../../src/main/core-options");
-const codeSamples = require("../../../website/playground/codeSamples").default;
+const { projectRoot } = require("../env.js");
+const coreOptions = require("../../../src/main/core-options.js");
+const codeSamples =
+  require("../../../website/playground/codeSamples.js").default;
 
 const parserNames = coreOptions.options.parser.choices.map(
   ({ value }) => value
diff --git a/tests/integration/__tests__/check.js b/tests/integration/__tests__/check.js
index 4a1e3eebd..e47eec277 100644
--- a/tests/integration/__tests__/check.js
+++ b/tests/integration/__tests__/check.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("checks stdin with --check", () => {
   runPrettier("cli/with-shebang", ["--check", "--parser", "babel"], {
diff --git a/tests/integration/__tests__/cli.js b/tests/integration/__tests__/cli.js
index b78919115..362b4e5d2 100644
--- a/tests/integration/__tests__/cli.js
+++ b/tests/integration/__tests__/cli.js
@@ -1,6 +1,6 @@
 "use strict";
 const fs = require("fs");
-const { prettierCli } = require("../env");
+const { prettierCli } = require("../env.js");
 
 describe("CLI", () => {
   test("CLI should be executable.", () => {
diff --git a/tests/integration/__tests__/config-invalid.js b/tests/integration/__tests__/config-invalid.js
index c94fbe96e..d8acde855 100644
--- a/tests/integration/__tests__/config-invalid.js
+++ b/tests/integration/__tests__/config-invalid.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("throw error for unsupported extension", () => {
   runPrettier("cli/config/invalid", [
diff --git a/tests/integration/__tests__/config-resolution.js b/tests/integration/__tests__/config-resolution.js
index 10f231ebe..7e39b7f8f 100644
--- a/tests/integration/__tests__/config-resolution.js
+++ b/tests/integration/__tests__/config-resolution.js
@@ -3,9 +3,9 @@
 const path = require("path");
 
 const prettier = require("prettier-local");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("resolves configuration from external files", () => {
   runPrettier("cli/config/", ["--end-of-line", "lf", "**/*.js"]).test({
diff --git a/tests/integration/__tests__/cursor-offset.js b/tests/integration/__tests__/cursor-offset.js
index 172febb45..e8c4f375b 100644
--- a/tests/integration/__tests__/cursor-offset.js
+++ b/tests/integration/__tests__/cursor-offset.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("write cursorOffset to stderr with --cursor-offset <int>", () => {
   runPrettier("cli", ["--cursor-offset", "2", "--parser", "babel"], {
diff --git a/tests/integration/__tests__/debug-check.js b/tests/integration/__tests__/debug-check.js
index 0e2a63c77..94a196d5f 100644
--- a/tests/integration/__tests__/debug-check.js
+++ b/tests/integration/__tests__/debug-check.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("doesn't crash when --debug-check is passed", () => {
   runPrettier("cli/with-shebang", ["issue1890.js", "--debug-check"]).test({
diff --git a/tests/integration/__tests__/debug-print-comments.js b/tests/integration/__tests__/debug-print-comments.js
index 73358ce26..5b10539f3 100644
--- a/tests/integration/__tests__/debug-print-comments.js
+++ b/tests/integration/__tests__/debug-print-comments.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("prints information for debugging comment attachment with --debug-print-comments", () => {
   runPrettier(
diff --git a/tests/integration/__tests__/debug-print-doc.js b/tests/integration/__tests__/debug-print-doc.js
index aa0995cdf..b165b6c93 100644
--- a/tests/integration/__tests__/debug-print-doc.js
+++ b/tests/integration/__tests__/debug-print-doc.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("prints doc with --debug-print-doc", () => {
   runPrettier("cli/with-shebang", ["--debug-print-doc", "--parser", "babel"], {
diff --git a/tests/integration/__tests__/early-exit.js b/tests/integration/__tests__/early-exit.js
index 3507dc223..a23db751a 100644
--- a/tests/integration/__tests__/early-exit.js
+++ b/tests/integration/__tests__/early-exit.js
@@ -1,8 +1,8 @@
 "use strict";
 
 const prettier = require("prettier-local");
-const runPrettier = require("../runPrettier");
-const { isProduction } = require("../env");
+const runPrettier = require("../runPrettier.js");
+const { isProduction } = require("../env.js");
 
 describe("show version with --version", () => {
   runPrettier("cli/with-shebang", ["--version"]).test({
diff --git a/tests/integration/__tests__/error-on-unmatched-pattern.js b/tests/integration/__tests__/error-on-unmatched-pattern.js
index 102f61326..0cea212d3 100644
--- a/tests/integration/__tests__/error-on-unmatched-pattern.js
+++ b/tests/integration/__tests__/error-on-unmatched-pattern.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("no error on unmatched pattern", () => {
   runPrettier("cli/error-on-unmatched-pattern", [
diff --git a/tests/integration/__tests__/file-info.js b/tests/integration/__tests__/file-info.js
index 5cd4ef1d8..009b36820 100644
--- a/tests/integration/__tests__/file-info.js
+++ b/tests/integration/__tests__/file-info.js
@@ -6,9 +6,9 @@ const tempy = require("tempy");
 const fromPairs = require("lodash/fromPairs");
 
 const prettier = require("prettier-local");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("extracts file-info for a js file", () => {
   runPrettier("cli/", ["--file-info", "something.js"]).test({
diff --git a/tests/integration/__tests__/format.js b/tests/integration/__tests__/format.js
index 6a83e86e6..50578fa4c 100644
--- a/tests/integration/__tests__/format.js
+++ b/tests/integration/__tests__/format.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const prettier = require("prettier-local");
-const fooPlugin = require("../plugins/defaultOptions/plugin");
+const fooPlugin = require("../plugins/defaultOptions/plugin.js");
 
 test("yaml parser should handle CRLF correctly", () => {
   const input = "a:\r\n  123\r\n";
diff --git a/tests/integration/__tests__/help-options.js b/tests/integration/__tests__/help-options.js
index 01c470402..e8a616ee5 100644
--- a/tests/integration/__tests__/help-options.js
+++ b/tests/integration/__tests__/help-options.js
@@ -1,10 +1,10 @@
 "use strict";
 
 const prettier = require("prettier-local");
-const runPrettier = require("../runPrettier");
-const constant = require("../../../src/cli/constant");
-const core = require("../../../src/cli/core");
-const arrayify = require("../../../src/utils/arrayify");
+const runPrettier = require("../runPrettier.js");
+const constant = require("../../../src/cli/constant.js");
+const core = require("../../../src/cli/core.js");
+const arrayify = require("../../../src/utils/arrayify.js");
 
 for (const option of arrayify(
   {
diff --git a/tests/integration/__tests__/ignore-absolute-path.js b/tests/integration/__tests__/ignore-absolute-path.js
index 5a7bdd6a6..79fde347f 100644
--- a/tests/integration/__tests__/ignore-absolute-path.js
+++ b/tests/integration/__tests__/ignore-absolute-path.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const path = require("path");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("support absolute filename", () => {
   runPrettier("cli/ignore-absolute-path", [
diff --git a/tests/integration/__tests__/ignore-emoji.js b/tests/integration/__tests__/ignore-emoji.js
index db25350a3..a18ff4e90 100644
--- a/tests/integration/__tests__/ignore-emoji.js
+++ b/tests/integration/__tests__/ignore-emoji.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("ignores file name contains emoji", () => {
   runPrettier("cli/ignore-emoji", ["**/*.js", "-l"]).test({
diff --git a/tests/integration/__tests__/ignore-in-subdirectories.js b/tests/integration/__tests__/ignore-in-subdirectories.js
index 4b67ffb02..b30d361fe 100644
--- a/tests/integration/__tests__/ignore-in-subdirectories.js
+++ b/tests/integration/__tests__/ignore-in-subdirectories.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("ignores files when executing in a subdirectory", () => {
   runPrettier("cli/ignore-in-subdirectories/web1", [
diff --git a/tests/integration/__tests__/ignore-path.js b/tests/integration/__tests__/ignore-path.js
index 9287891a9..b7877a1bd 100644
--- a/tests/integration/__tests__/ignore-path.js
+++ b/tests/integration/__tests__/ignore-path.js
@@ -2,7 +2,7 @@
 
 const fs = require("fs");
 const path = require("path");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 fs.writeFileSync(
   // This file is in `.gitignore`, just copy from `regular-module.js`
diff --git a/tests/integration/__tests__/ignore-relative-path.js b/tests/integration/__tests__/ignore-relative-path.js
index d48bd0b5d..85a0939cd 100644
--- a/tests/integration/__tests__/ignore-relative-path.js
+++ b/tests/integration/__tests__/ignore-relative-path.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("support relative paths", () => {
   runPrettier("cli/ignore-relative-path", [
diff --git a/tests/integration/__tests__/ignore-unknown.js b/tests/integration/__tests__/ignore-unknown.js
index 01b5581e2..13846a720 100644
--- a/tests/integration/__tests__/ignore-unknown.js
+++ b/tests/integration/__tests__/ignore-unknown.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("ignore-unknown dir", () => {
   runPrettier("cli/ignore-unknown", [
diff --git a/tests/integration/__tests__/ignore-vcs-files.js b/tests/integration/__tests__/ignore-vcs-files.js
index 064af6ec5..058bd15f6 100644
--- a/tests/integration/__tests__/ignore-vcs-files.js
+++ b/tests/integration/__tests__/ignore-vcs-files.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("ignores files in version control systems", () => {
   runPrettier("cli/ignore-vcs-files", [
diff --git a/tests/integration/__tests__/infer-parser.js b/tests/integration/__tests__/infer-parser.js
index 88ea30065..094a90fd7 100644
--- a/tests/integration/__tests__/infer-parser.js
+++ b/tests/integration/__tests__/infer-parser.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const prettier = require("prettier-local");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("stdin no path and no parser", () => {
   describe("logs error and exits with 2", () => {
diff --git a/tests/integration/__tests__/invalid-ignore.js b/tests/integration/__tests__/invalid-ignore.js
index e7f7aa869..55ed1cc7a 100644
--- a/tests/integration/__tests__/invalid-ignore.js
+++ b/tests/integration/__tests__/invalid-ignore.js
@@ -2,9 +2,9 @@
 
 const path = require("path");
 const prettier = require("prettier-local");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("throw error with invalid ignore", () => {
   runPrettier("cli/invalid-ignore", ["something.js"]).test({
diff --git a/tests/integration/__tests__/line-suffix-boundary.js b/tests/integration/__tests__/line-suffix-boundary.js
index 8622a9bce..8e1cd7a03 100644
--- a/tests/integration/__tests__/line-suffix-boundary.js
+++ b/tests/integration/__tests__/line-suffix-boundary.js
@@ -6,7 +6,7 @@ const prettier = require("prettier-local");
 const { group, indent, line, lineSuffix, lineSuffixBoundary, softline } =
   prettier.doc.builders;
 
-const printDoc = require("../printDoc");
+const printDoc = require("../printDoc.js");
 
 describe("lineSuffixBoundary", () => {
   test("should be correctly treated as a potential line break in `fits`", () => {
diff --git a/tests/integration/__tests__/list-different.js b/tests/integration/__tests__/list-different.js
index c767e6d4d..2de416d56 100644
--- a/tests/integration/__tests__/list-different.js
+++ b/tests/integration/__tests__/list-different.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("checks stdin with --list-different", () => {
   runPrettier("cli/with-shebang", ["--list-different", "--parser", "babel"], {
diff --git a/tests/integration/__tests__/load-toml.js b/tests/integration/__tests__/load-toml.js
index 8c113058e..fb92b1cf9 100644
--- a/tests/integration/__tests__/load-toml.js
+++ b/tests/integration/__tests__/load-toml.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const loadToml = require("../../../src/utils/load-toml");
+const loadToml = require("../../../src/utils/load-toml.js");
 
 describe("TOML", () => {
   const exampleFilePath = "example.toml";
diff --git a/tests/integration/__tests__/loglevel.js b/tests/integration/__tests__/loglevel.js
index 54b854dc4..e7a0671be 100644
--- a/tests/integration/__tests__/loglevel.js
+++ b/tests/integration/__tests__/loglevel.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const stripAnsi = require("strip-ansi");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 test("do not show logs with --loglevel silent", async () => {
   await runPrettierWithLogLevel("silent", null);
diff --git a/tests/integration/__tests__/parser-api.js b/tests/integration/__tests__/parser-api.js
index f3ed30025..0bdf03690 100644
--- a/tests/integration/__tests__/parser-api.js
+++ b/tests/integration/__tests__/parser-api.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const prettier = require("prettier-local");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 test("allows custom parser provided as object", () => {
   const output = prettier.format("1", {
diff --git a/tests/integration/__tests__/patterns-dirs.js b/tests/integration/__tests__/patterns-dirs.js
index 5f4666ca9..803b41377 100644
--- a/tests/integration/__tests__/patterns-dirs.js
+++ b/tests/integration/__tests__/patterns-dirs.js
@@ -2,10 +2,10 @@
 
 const path = require("path");
 const fs = require("fs");
-const runPrettier = require("../runPrettier");
-const { projectRoot } = require("../env");
+const runPrettier = require("../runPrettier.js");
+const { projectRoot } = require("../env.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 // ESLint-like behavior
 // https://github.com/prettier/prettier/pull/6639#issuecomment-548949954
diff --git a/tests/integration/__tests__/patterns-glob.js b/tests/integration/__tests__/patterns-glob.js
index 430a62a59..4bb648939 100644
--- a/tests/integration/__tests__/patterns-glob.js
+++ b/tests/integration/__tests__/patterns-glob.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 /*
 fixtures-1/
diff --git a/tests/integration/__tests__/patterns.js b/tests/integration/__tests__/patterns.js
index 807f190c8..455829833 100644
--- a/tests/integration/__tests__/patterns.js
+++ b/tests/integration/__tests__/patterns.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("multiple patterns", () => {
   runPrettier("cli/patterns", [
diff --git a/tests/integration/__tests__/piped-output.js b/tests/integration/__tests__/piped-output.js
index 81604e21a..ad0cc248d 100644
--- a/tests/integration/__tests__/piped-output.js
+++ b/tests/integration/__tests__/piped-output.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("output with --check + unformatted differs when piped", () => {
   const result0 = runPrettier(
diff --git a/tests/integration/__tests__/plugin-default-options.js b/tests/integration/__tests__/plugin-default-options.js
index b0a3f24b9..76c9f4002 100644
--- a/tests/integration/__tests__/plugin-default-options.js
+++ b/tests/integration/__tests__/plugin-default-options.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("plugin default options should work", () => {
   runPrettier(
diff --git a/tests/integration/__tests__/plugin-extensions.js b/tests/integration/__tests__/plugin-extensions.js
index 26843f1b3..2bba45602 100644
--- a/tests/integration/__tests__/plugin-extensions.js
+++ b/tests/integration/__tests__/plugin-extensions.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 const EOL = "\n";
 
 describe("uses 'extensions' from languages to determine parser", () => {
diff --git a/tests/integration/__tests__/plugin-flush-line-suffix.js b/tests/integration/__tests__/plugin-flush-line-suffix.js
index e60ab5adb..0ed261439 100644
--- a/tests/integration/__tests__/plugin-flush-line-suffix.js
+++ b/tests/integration/__tests__/plugin-flush-line-suffix.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("flush all line-suffix content", () => {
   runPrettier("plugins/flushLineSuffix", ["*.foo", "--plugin=./plugin"], {
diff --git a/tests/integration/__tests__/plugin-options-string.js b/tests/integration/__tests__/plugin-options-string.js
index 4d3a83dbd..1dd3a4d88 100644
--- a/tests/integration/__tests__/plugin-options-string.js
+++ b/tests/integration/__tests__/plugin-options-string.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const snapshotDiff = require("snapshot-diff");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 test("show external options with `--help`", async () => {
   const originalStdout = await runPrettier("plugins/options-string", ["--help"])
diff --git a/tests/integration/__tests__/plugin-options.js b/tests/integration/__tests__/plugin-options.js
index 39aa86b60..22cafb738 100644
--- a/tests/integration/__tests__/plugin-options.js
+++ b/tests/integration/__tests__/plugin-options.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const snapshotDiff = require("snapshot-diff");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 test("show external options with `--help`", async () => {
   const originalStdout = await runPrettier("plugins/options", ["--help"])
diff --git a/tests/integration/__tests__/plugin-precedence.js b/tests/integration/__tests__/plugin-precedence.js
index 355b24a48..0770ab4b5 100644
--- a/tests/integration/__tests__/plugin-precedence.js
+++ b/tests/integration/__tests__/plugin-precedence.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("json-stringify takes precedence over json for package.json", () => {
   runPrettier("plugins", ["--stdin-filepath=package.json"], {
diff --git a/tests/integration/__tests__/plugin-preprocess.js b/tests/integration/__tests__/plugin-preprocess.js
index 23819fe50..0c0452e42 100644
--- a/tests/integration/__tests__/plugin-preprocess.js
+++ b/tests/integration/__tests__/plugin-preprocess.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 const EOL = "\n";
 
 describe("parser preprocess function is used to reshape input text", () => {
diff --git a/tests/integration/__tests__/plugin-resolution.js b/tests/integration/__tests__/plugin-resolution.js
index bcd2ec785..8b51b7866 100644
--- a/tests/integration/__tests__/plugin-resolution.js
+++ b/tests/integration/__tests__/plugin-resolution.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 const EOL = "\n";
 
 describe("automatically loads 'prettier-plugin-*'", () => {
diff --git a/tests/integration/__tests__/plugin-virtual-directory.js b/tests/integration/__tests__/plugin-virtual-directory.js
index 4602eb013..44c95f6c4 100644
--- a/tests/integration/__tests__/plugin-virtual-directory.js
+++ b/tests/integration/__tests__/plugin-virtual-directory.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("plugin search should not crash when prettier isn't inside a directory", () => {
   runPrettier(
diff --git a/tests/integration/__tests__/schema.js b/tests/integration/__tests__/schema.js
index e139809d0..2fdddde97 100644
--- a/tests/integration/__tests__/schema.js
+++ b/tests/integration/__tests__/schema.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const prettier = require("prettier-local");
-const generateSchema = require("../../../scripts/utils/generate-schema");
+const generateSchema = require("../../../scripts/utils/generate-schema.js");
 
 test("schema", () => {
   expect(generateSchema(prettier.getSupportInfo().options)).toMatchSnapshot();
diff --git a/tests/integration/__tests__/skip-folders.js b/tests/integration/__tests__/skip-folders.js
index 193781c9e..dd1ea822b 100644
--- a/tests/integration/__tests__/skip-folders.js
+++ b/tests/integration/__tests__/skip-folders.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("skips folders in glob", () => {
   runPrettier("cli/skip-folders", ["**/*", "-l"]).test({
diff --git a/tests/integration/__tests__/stdin-filepath.js b/tests/integration/__tests__/stdin-filepath.js
index 32cb97e23..53f53b8e1 100644
--- a/tests/integration/__tests__/stdin-filepath.js
+++ b/tests/integration/__tests__/stdin-filepath.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const { isCI } = require("ci-info");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("format correctly if stdin content compatible with stdin-filepath", () => {
   runPrettier(
diff --git a/tests/integration/__tests__/support-info.js b/tests/integration/__tests__/support-info.js
index 3886c1f33..57cd9d7df 100644
--- a/tests/integration/__tests__/support-info.js
+++ b/tests/integration/__tests__/support-info.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const prettier = require("prettier-local");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 test("API getSupportInfo()", () => {
   expect(getCoreInfo()).toMatchSnapshot();
diff --git a/tests/integration/__tests__/syntax-error.js b/tests/integration/__tests__/syntax-error.js
index 362ffab40..e33c23e5b 100644
--- a/tests/integration/__tests__/syntax-error.js
+++ b/tests/integration/__tests__/syntax-error.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("exits with non-zero code when input has a syntax error", () => {
   runPrettier("cli/with-shebang", ["--parser", "babel"], {
diff --git a/tests/integration/__tests__/third-party.js b/tests/integration/__tests__/third-party.js
index 8dcb6079a..8cf5f2a31 100644
--- a/tests/integration/__tests__/third-party.js
+++ b/tests/integration/__tests__/third-party.js
@@ -1,10 +1,10 @@
 "use strict";
 
 const path = require("path");
-const { thirdParty } = require("../env");
+const { thirdParty } = require("../env.js");
 const { cosmiconfig, cosmiconfigSync, isCI } = require(thirdParty);
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 // This don't has to be the same result as `prettier.resolveConfig`,
 // Because we are testing with default `cosmiconfigOptions`
diff --git a/tests/integration/__tests__/util-shared.js b/tests/integration/__tests__/util-shared.js
index 5699c9675..764487ec2 100644
--- a/tests/integration/__tests__/util-shared.js
+++ b/tests/integration/__tests__/util-shared.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const sharedUtil = require("../../../src/common/util-shared");
+const sharedUtil = require("../../../src/common/util-shared.js");
 
 test("shared util has correct structure", () => {
   expect(typeof sharedUtil.getMaxContinuousCount).toEqual("function");
diff --git a/tests/integration/__tests__/with-config-precedence.js b/tests/integration/__tests__/with-config-precedence.js
index 22a2e69b9..39944e10f 100644
--- a/tests/integration/__tests__/with-config-precedence.js
+++ b/tests/integration/__tests__/with-config-precedence.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("CLI overrides take precedence without --config-precedence", () => {
   runPrettier("cli/config/", [
diff --git a/tests/integration/__tests__/with-node-modules.js b/tests/integration/__tests__/with-node-modules.js
index 212d80d5c..689b9eafa 100644
--- a/tests/integration/__tests__/with-node-modules.js
+++ b/tests/integration/__tests__/with-node-modules.js
@@ -1,8 +1,8 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
-expect.addSnapshotSerializer(require("../path-serializer"));
+expect.addSnapshotSerializer(require("../path-serializer.js"));
 
 describe("ignores node_modules by default", () => {
   runPrettier("cli/with-node-modules", ["**/*.js", "-l"]).test({
diff --git a/tests/integration/__tests__/with-parser-inference.js b/tests/integration/__tests__/with-parser-inference.js
index 020005d12..4e0c0cb3f 100644
--- a/tests/integration/__tests__/with-parser-inference.js
+++ b/tests/integration/__tests__/with-parser-inference.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const prettier = require("prettier-local");
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("infers postcss parser", () => {
   runPrettier("cli/with-parser-inference", ["--end-of-line", "lf", "*"]).test({
diff --git a/tests/integration/__tests__/with-shebang.js b/tests/integration/__tests__/with-shebang.js
index 62c78f744..7d9e7bae7 100644
--- a/tests/integration/__tests__/with-shebang.js
+++ b/tests/integration/__tests__/with-shebang.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("preserves shebang", () => {
   runPrettier("cli/with-shebang", ["--end-of-line", "lf", "issue1890.js"]).test(
diff --git a/tests/integration/__tests__/write.js b/tests/integration/__tests__/write.js
index 32d891a38..a649af391 100644
--- a/tests/integration/__tests__/write.js
+++ b/tests/integration/__tests__/write.js
@@ -1,6 +1,6 @@
 "use strict";
 
-const runPrettier = require("../runPrettier");
+const runPrettier = require("../runPrettier.js");
 
 describe("write file with --write + unformatted file", () => {
   runPrettier("cli/write", ["--write", "unformatted.js"]).test({
diff --git a/tests/integration/runPrettier.js b/tests/integration/runPrettier.js
index 3f0e0b62c..e92bd270e 100644
--- a/tests/integration/runPrettier.js
+++ b/tests/integration/runPrettier.js
@@ -3,7 +3,7 @@
 const fs = require("fs");
 const path = require("path");
 const stripAnsi = require("strip-ansi");
-const { prettierCli, thirdParty } = require("./env");
+const { prettierCli, thirdParty } = require("./env.js");
 
 async function run(dir, args, options) {
   args = Array.isArray(args) ? args : [args];
diff --git a/website/pages/en/versions.js b/website/pages/en/versions.js
index 0ac98cdc4..e3c349e69 100755
--- a/website/pages/en/versions.js
+++ b/website/pages/en/versions.js
@@ -8,7 +8,7 @@
  */
 
 const React = require("react");
-const { Container } = require("../../core/CompLibrary");
+const { Container } = require("../../core/CompLibrary.js");
 
 const CWD = process.cwd();
 
diff --git a/website/playground/EditorState.js b/website/playground/EditorState.js
index 7ec595a22..69c6de097 100644
--- a/website/playground/EditorState.js
+++ b/website/playground/EditorState.js
@@ -1,7 +1,7 @@
 import * as React from "react";
 
-import { stateToggler, shallowEqual } from "./helpers";
-import * as storage from "./storage";
+import { stateToggler, shallowEqual } from "./helpers.js";
+import * as storage from "./storage.js";
 
 export default class extends React.Component {
   constructor() {
diff --git a/website/playground/Playground.js b/website/playground/Playground.js
index 0892dff1c..e293054d9 100644
--- a/website/playground/Playground.js
+++ b/website/playground/Playground.js
@@ -1,19 +1,19 @@
 import * as React from "react";
 
-import { Button, ClipboardButton } from "./buttons";
-import EditorState from "./EditorState";
-import { DebugPanel, InputPanel, OutputPanel } from "./panels";
-import PrettierFormat from "./PrettierFormat";
-import { shallowEqual } from "./helpers";
-import * as urlHash from "./urlHash";
-import formatMarkdown from "./markdown";
-import * as util from "./util";
-import getCodeSample from "./codeSamples";
+import { Button, ClipboardButton } from "./buttons.js";
+import EditorState from "./EditorState.js";
+import { DebugPanel, InputPanel, OutputPanel } from "./panels.js";
+import PrettierFormat from "./PrettierFormat.js";
+import { shallowEqual } from "./helpers.js";
+import * as urlHash from "./urlHash.js";
+import formatMarkdown from "./markdown.js";
+import * as util from "./util.js";
+import getCodeSample from "./codeSamples.js";
 
-import { Sidebar, SidebarCategory } from "./sidebar/components";
-import SidebarOptions from "./sidebar/SidebarOptions";
-import Option from "./sidebar/options";
-import { Checkbox } from "./sidebar/inputs";
+import { Sidebar, SidebarCategory } from "./sidebar/components.js";
+import SidebarOptions from "./sidebar/SidebarOptions.js";
+import Option from "./sidebar/options.js";
+import { Checkbox } from "./sidebar/inputs.js";
 
 const CATEGORIES_ORDER = [
   "Global",
diff --git a/website/playground/index.js b/website/playground/index.js
index fe847770b..67d42705a 100644
--- a/website/playground/index.js
+++ b/website/playground/index.js
@@ -3,10 +3,10 @@ import "codemirror-graphql/mode";
 import * as React from "react";
 import * as ReactDOM from "react-dom";
 
-import Playground from "./Playground";
-import VersionLink from "./VersionLink";
-import WorkerApi from "./WorkerApi";
-import { fixPrettierVersion } from "./util";
+import Playground from "./Playground.js";
+import VersionLink from "./VersionLink.js";
+import WorkerApi from "./WorkerApi.js";
+import { fixPrettierVersion } from "./util.js";
 
 class App extends React.Component {
   constructor() {
diff --git a/website/playground/sidebar/SidebarOptions.js b/website/playground/sidebar/SidebarOptions.js
index c6b8e080a..09ed4985b 100644
--- a/website/playground/sidebar/SidebarOptions.js
+++ b/website/playground/sidebar/SidebarOptions.js
@@ -1,8 +1,8 @@
 import * as React from "react";
 import groupBy from "lodash/groupBy";
 
-import { SidebarCategory } from "./components";
-import Option from "./options";
+import { SidebarCategory } from "./components.js";
+import Option from "./options.js";
 
 export default function ({
   categories,
diff --git a/website/playground/sidebar/options.js b/website/playground/sidebar/options.js
index 8c81e4622..376f6e713 100644
--- a/website/playground/sidebar/options.js
+++ b/website/playground/sidebar/options.js
@@ -1,6 +1,6 @@
 import * as React from "react";
 
-import { Checkbox, Select, NumberInput } from "./inputs";
+import { Checkbox, Select, NumberInput } from "./inputs.js";
 
 export function BooleanOption({ option, value, onChange }) {
   function maybeInvert(value) {