From 72cfa18a42e2d4cd3a41ad46c7742701b17c91c5 Mon Sep 17 00:00:00 2001
From: Konv Suu <2583695112@qq.com>
Date: Wed, 24 Sep 2025 01:29:56 +0800
Subject: [PATCH] Support format html event attributes (#17909)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: fisker <lionkay@gmail.com>
---
 changelog_unreleased/html/17909.md            | 26 ++++++
 package.json                                  |  1 +
 src/language-html/embed/attribute.js          |  2 +
 src/language-html/embed/class-names.js        | 12 ++-
 src/language-html/embed/event-handler.js      | 27 ++++++
 src/language-html/embed/style.js              | 20 ++--
 src/language-js/print/expression-statement.js | 34 ++++---
 src/language-js/print/semicolon.js            | 29 ++++--
 .../__snapshots__/format.test.js.snap         | 92 +++++++++++++++++++
 .../html/attributes/event-attributes.html     | 32 +++++++
 .../no-semi/__snapshots__/format.test.js.snap | 27 ++++++
 .../attributes/no-semi/event-attributes.html  |  6 ++
 .../html/attributes/no-semi/format.test.js    |  1 +
 .../tags/__snapshots__/format.test.js.snap    | 12 ++-
 yarn.lock                                     |  8 ++
 15 files changed, 291 insertions(+), 38 deletions(-)
 create mode 100644 changelog_unreleased/html/17909.md
 create mode 100644 src/language-html/embed/event-handler.js
 create mode 100644 tests/format/html/attributes/event-attributes.html
 create mode 100644 tests/format/html/attributes/no-semi/__snapshots__/format.test.js.snap
 create mode 100644 tests/format/html/attributes/no-semi/event-attributes.html
 create mode 100644 tests/format/html/attributes/no-semi/format.test.js

diff --git a/changelog_unreleased/html/17909.md b/changelog_unreleased/html/17909.md
new file mode 100644
index 000000000..88c5a5bcb
--- /dev/null
+++ b/changelog_unreleased/html/17909.md
@@ -0,0 +1,26 @@
+#### Format inline event handler (#17909 by @kovsu)
+
+<!-- prettier-ignore -->
+```html
+<!-- Input -->
+<button
+  type="button"
+  onclick="console .log(    'Hello, this is my old-fashioned event handler!')"
+>Press me</button>
+
+<!-- Prettier stable -->
+<button
+  type="button"
+  onclick="console .log(    'Hello, this is my old-fashioned event handler!')"
+>
+  Press me
+</button>
+
+<!-- Prettier main -->
+<button
+  type="button"
+  onclick="console.log('Hello, this is my old-fashioned event handler!')"
+>
+  Press me
+</button>
+```
diff --git a/package.json b/package.json
index 412335db0..bc5b35d28 100644
--- a/package.json
+++ b/package.json
@@ -38,6 +38,7 @@
     "@babel/types": "7.28.4",
     "@glimmer/syntax": "0.95.0",
     "@prettier/cli": "0.9.0",
+    "@prettier/html-event-attributes": "0.0.1",
     "@prettier/html-tags": "1.1.0",
     "@prettier/parse-srcset": "3.1.0",
     "@typescript-eslint/typescript-estree": "8.44.1",
diff --git a/src/language-html/embed/attribute.js b/src/language-html/embed/attribute.js
index 0adbe272b..718bfc8b6 100644
--- a/src/language-html/embed/attribute.js
+++ b/src/language-html/embed/attribute.js
@@ -2,6 +2,7 @@ import { group } from "../../document/builders.js";
 import { mapDoc } from "../../document/utils.js";
 import printAngularAttribute from "./angular-attributes.js";
 import printClassNames from "./class-names.js";
+import printEventHandler from "./event-handler.js";
 import printSrcset from "./srcset.js";
 import { printStyleAttribute } from "./style.js";
 import printVueAttribute from "./vue-attributes.js";
@@ -36,6 +37,7 @@ function printAttribute(path, options) {
   for (const getValuePrinter of [
     printSrcset,
     printStyleAttribute,
+    printEventHandler,
     printClassNames,
     printVueAttribute,
     printAngularAttribute,
diff --git a/src/language-html/embed/class-names.js b/src/language-html/embed/class-names.js
index 029e26ced..31a3ebc4b 100644
--- a/src/language-html/embed/class-names.js
+++ b/src/language-html/embed/class-names.js
@@ -2,14 +2,16 @@ import { getUnescapedAttributeValue } from "../utils/index.js";
 
 function printClassNames(path, options) {
   const { node } = path;
-  const value = getUnescapedAttributeValue(node);
+
   if (
-    node.fullName === "class" &&
-    !options.parentParser &&
-    !value.includes("{{")
+    options.parentParser ||
+    node.fullName !== "class" ||
+    node.value.includes("{{")
   ) {
-    return () => value.trim().split(/\s+/u).join(" ");
+    return;
   }
+
+  return () => getUnescapedAttributeValue(node).trim().split(/\s+/u).join(" ");
 }
 
 export default printClassNames;
diff --git a/src/language-html/embed/event-handler.js b/src/language-html/embed/event-handler.js
new file mode 100644
index 000000000..f1fd53eb2
--- /dev/null
+++ b/src/language-html/embed/event-handler.js
@@ -0,0 +1,27 @@
+import htmlEventAttributesArray from "@prettier/html-event-attributes";
+import { getUnescapedAttributeValue } from "../utils/index.js";
+import { formatAttributeValue } from "./utils.js";
+
+const htmlEventAttributes = new Set(htmlEventAttributesArray);
+
+function printEventHandler(path, options) {
+  const { node } = path;
+
+  if (
+    options.parentParser ||
+    !htmlEventAttributes.has(node.fullName) ||
+    node.value.includes("{{")
+  ) {
+    return;
+  }
+
+  return (textToDoc) =>
+    formatAttributeValue(
+      getUnescapedAttributeValue(node),
+      textToDoc,
+      { parser: "babel", __isHtmlInlineEventHandler: true },
+      () => false,
+    );
+}
+
+export default printEventHandler;
diff --git a/src/language-html/embed/style.js b/src/language-html/embed/style.js
index 544a9ea0d..f07bee2c3 100644
--- a/src/language-html/embed/style.js
+++ b/src/language-html/embed/style.js
@@ -3,17 +3,21 @@ import { printExpand } from "./utils.js";
 
 function printStyleAttribute(path, options) {
   const { node } = path;
-  const text = getUnescapedAttributeValue(path.node).trim();
   if (
-    node.fullName === "style" &&
-    !options.parentParser &&
-    !text.includes("{{")
+    node.fullName !== "style" ||
+    options.parentParser ||
+    node.value.includes("{{")
   ) {
-    return async (textToDoc) =>
-      printExpand(
-        await textToDoc(text, { parser: "css", __isHTMLStyleAttribute: true }),
-      );
+    return;
   }
+
+  return async (textToDoc) =>
+    printExpand(
+      await textToDoc(getUnescapedAttributeValue(node), {
+        parser: "css",
+        __isHTMLStyleAttribute: true,
+      }),
+    );
 }
 
 export { printStyleAttribute };
diff --git a/src/language-js/print/expression-statement.js b/src/language-js/print/expression-statement.js
index 97de0d7d7..aa0f9cad1 100644
--- a/src/language-js/print/expression-statement.js
+++ b/src/language-js/print/expression-statement.js
@@ -4,28 +4,38 @@ import {
   unwrapVueEventBindingTsNode,
 } from "../utils/vue-event-binding.js";
 import {
+  isSingleHtmlEventHandlerExpressionStatement,
   isSingleJsxExpressionStatementInMarkdown,
   isSingleVueEventBindingExpressionStatement,
 } from "./semicolon.js";
 
-function printExpressionStatement(path, options, print) {
-  const parts = [print("expression")];
-
+function shouldPrintSemicolon(path, options) {
   if (isSingleVueEventBindingExpressionStatement(path, options)) {
     const expression = unwrapVueEventBindingTsNode(path.node.expression);
-    if (
+    return (
       isVueEventBindingFunctionExpression(expression) ||
       isVueEventBindingMemberExpression(expression)
-    ) {
-      parts.push(";");
-    }
-  } else if (isSingleJsxExpressionStatementInMarkdown(path, options)) {
-    // Do not append semicolon after the only JSX element in a program
-  } else if (options.semi) {
-    parts.push(";");
+    );
   }
 
-  return parts;
+  if (!options.semi) {
+    return false;
+  }
+
+  if (
+    // Do not append semicolon after the only JSX element in a program
+    isSingleJsxExpressionStatementInMarkdown(path, options) ||
+    // Do not append semicolon after the only HTML event binding expression in a program
+    isSingleHtmlEventHandlerExpressionStatement(path, options)
+  ) {
+    return false;
+  }
+
+  return true;
+}
+
+function printExpressionStatement(path, options, print) {
+  return [print("expression"), shouldPrintSemicolon(path, options) ? ";" : ""];
 }
 
 export { printExpressionStatement };
diff --git a/src/language-js/print/semicolon.js b/src/language-js/print/semicolon.js
index 76b1f0b7a..0771b436e 100644
--- a/src/language-js/print/semicolon.js
+++ b/src/language-js/print/semicolon.js
@@ -10,7 +10,8 @@ function shouldPrintLeadingSemicolon(path, options) {
   if (
     options.semi ||
     isSingleJsxExpressionStatementInMarkdown(path, options) ||
-    isSingleVueEventBindingExpressionStatement(path, options)
+    isSingleVueEventBindingExpressionStatement(path, options) ||
+    isSingleHtmlEventHandlerExpressionStatement(path, options)
   ) {
     return false;
   }
@@ -89,27 +90,35 @@ function expressionNeedsASIProtection(path, options) {
   );
 }
 
-function isSingleJsxExpressionStatementInMarkdown({ node, parent }, options) {
+const isSingleExpressionStatement = ({ node, parent }) =>
+  node.type === "ExpressionStatement" &&
+  parent.type === "Program" &&
+  parent.body.length === 1;
+
+function isSingleJsxExpressionStatementInMarkdown(path, options) {
   return (
     (options.parentParser === "markdown" || options.parentParser === "mdx") &&
-    node.type === "ExpressionStatement" &&
-    isJsxElement(node.expression) &&
-    parent.type === "Program" &&
-    parent.body.length === 1
+    isSingleExpressionStatement(path) &&
+    isJsxElement(path.node.expression)
   );
 }
 
-function isSingleVueEventBindingExpressionStatement({ node, parent }, options) {
+function isSingleHtmlEventHandlerExpressionStatement(path, options) {
+  return (
+    options.__isHtmlInlineEventHandler && isSingleExpressionStatement(path)
+  );
+}
+
+function isSingleVueEventBindingExpressionStatement(path, options) {
   return (
     (options.parser === "__vue_event_binding" ||
       options.parser === "__vue_ts_event_binding") &&
-    node.type === "ExpressionStatement" &&
-    parent.type === "Program" &&
-    parent.body.length === 1
+    isSingleExpressionStatement(path)
   );
 }
 
 export {
+  isSingleHtmlEventHandlerExpressionStatement,
   isSingleJsxExpressionStatementInMarkdown,
   isSingleVueEventBindingExpressionStatement,
   shouldPrintLeadingSemicolon,
diff --git a/tests/format/html/attributes/__snapshots__/format.test.js.snap b/tests/format/html/attributes/__snapshots__/format.test.js.snap
index cb612b9f7..04f8fd966 100644
--- a/tests/format/html/attributes/__snapshots__/format.test.js.snap
+++ b/tests/format/html/attributes/__snapshots__/format.test.js.snap
@@ -537,6 +537,98 @@ printWidth: 80
 ================================================================================
 `;
 
+exports[`event-attributes.html format 1`] = `
+====================================options=====================================
+parsers: ["html"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+<button onclick="alert(    '1')">click me!</button>
+
+<div onclick="handleClick(  event  )" onmouseover="showTooltip( this )" onmouseout="hideTooltip(  )">Hover me</div>
+
+<img onerror='handleError(  this  )' onload="imageLoaded(  event  )" />
+
+<button onclick="(event) =>   {  console.log('clicked');  handleClick(  event  );  }">Modern JS</button>
+
+<div onclick="processData(  { id: 123, name: 'test'  },  [ 1,  2, 3 ]  )">Data</div>
+
+<button onclick="api.user.get(  123  ).then(  data =>   updateUI( data )  )">API Call</button>
+
+<input onfocus="highlightField(  this  )" onblur="validateField( this.value )" />
+<select onchange="filterResults(  this.value,  this.selectedIndex  )">
+  <option>Option 1</option>
+</select>
+
+<button onclick="
+   alert(1);           alert(2);
+ ">Click</button>
+
+<button onclick="
+    return false
+  ">Click</button>
+
+<button onclick="
+  console.log(arguments)
+  ">Click</button>
+
+<button onclick="
+  console.log(onclick)
+  ">Click</button>
+
+=====================================output=====================================
+<button onclick="alert('1')">click me!</button>
+
+<div
+  onclick="handleClick(event)"
+  onmouseover="showTooltip(this)"
+  onmouseout="hideTooltip()"
+>
+  Hover me
+</div>
+
+<img onerror="handleError(this)" onload="imageLoaded(event)" />
+
+<button
+  onclick="
+    (event) => {
+      console.log('clicked');
+      handleClick(event);
+    }
+  "
+>
+  Modern JS
+</button>
+
+<div onclick="processData({ id: 123, name: 'test' }, [1, 2, 3])">Data</div>
+
+<button onclick="api.user.get(123).then((data) => updateUI(data))">
+  API Call
+</button>
+
+<input onfocus="highlightField(this)" onblur="validateField(this.value)" />
+<select onchange="filterResults(this.value, this.selectedIndex)">
+  <option>Option 1</option>
+</select>
+
+<button
+  onclick="
+    alert(1);
+    alert(2);
+  "
+>
+  Click
+</button>
+
+<button onclick="return false;">Click</button>
+
+<button onclick="console.log(arguments)">Click</button>
+
+<button onclick="console.log(onclick)">Click</button>
+
+================================================================================
+`;
+
 exports[`single-quotes.html format 1`] = `
 ====================================options=====================================
 parsers: ["html"]
diff --git a/tests/format/html/attributes/event-attributes.html b/tests/format/html/attributes/event-attributes.html
new file mode 100644
index 000000000..a3d256a2b
--- /dev/null
+++ b/tests/format/html/attributes/event-attributes.html
@@ -0,0 +1,32 @@
+<button onclick="alert(    '1')">click me!</button>
+
+<div onclick="handleClick(  event  )" onmouseover="showTooltip( this )" onmouseout="hideTooltip(  )">Hover me</div>
+
+<img onerror='handleError(  this  )' onload="imageLoaded(  event  )" />
+
+<button onclick="(event) =>   {  console.log('clicked');  handleClick(  event  );  }">Modern JS</button>
+
+<div onclick="processData(  { id: 123, name: 'test'  },  [ 1,  2, 3 ]  )">Data</div>
+
+<button onclick="api.user.get(  123  ).then(  data =>   updateUI( data )  )">API Call</button>
+
+<input onfocus="highlightField(  this  )" onblur="validateField( this.value )" />
+<select onchange="filterResults(  this.value,  this.selectedIndex  )">
+  <option>Option 1</option>
+</select>
+
+<button onclick="
+   alert(1);           alert(2);
+ ">Click</button>
+
+<button onclick="
+    return false
+  ">Click</button>
+
+<button onclick="
+  console.log(arguments)
+  ">Click</button>
+
+<button onclick="
+  console.log(onclick)
+  ">Click</button>
diff --git a/tests/format/html/attributes/no-semi/__snapshots__/format.test.js.snap b/tests/format/html/attributes/no-semi/__snapshots__/format.test.js.snap
new file mode 100644
index 000000000..c522795d8
--- /dev/null
+++ b/tests/format/html/attributes/no-semi/__snapshots__/format.test.js.snap
@@ -0,0 +1,27 @@
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+
+exports[`event-attributes.html - {"semi":false} format 1`] = `
+====================================options=====================================
+parsers: ["html"]
+printWidth: 80
+semi: false
+                                                                                | printWidth
+=====================================input======================================
+<button
+  onclick="[foo, bar].forEach((fn) => void fn())"
+></button>
+<button
+  onclick="[foo, bar].forEach((fn) => void fn()); alert(1)"
+></button>
+
+=====================================output=====================================
+<button onclick="[foo, bar].forEach((fn) => void fn())"></button>
+<button
+  onclick="
+    ;[foo, bar].forEach((fn) => void fn())
+    alert(1)
+  "
+></button>
+
+================================================================================
+`;
diff --git a/tests/format/html/attributes/no-semi/event-attributes.html b/tests/format/html/attributes/no-semi/event-attributes.html
new file mode 100644
index 000000000..51274fad0
--- /dev/null
+++ b/tests/format/html/attributes/no-semi/event-attributes.html
@@ -0,0 +1,6 @@
+<button
+  onclick="[foo, bar].forEach((fn) => void fn())"
+></button>
+<button
+  onclick="[foo, bar].forEach((fn) => void fn()); alert(1)"
+></button>
diff --git a/tests/format/html/attributes/no-semi/format.test.js b/tests/format/html/attributes/no-semi/format.test.js
new file mode 100644
index 000000000..04fa2b56d
--- /dev/null
+++ b/tests/format/html/attributes/no-semi/format.test.js
@@ -0,0 +1 @@
+runFormatTest(import.meta, ["html"], { semi: false });
diff --git a/tests/format/html/tags/__snapshots__/format.test.js.snap b/tests/format/html/tags/__snapshots__/format.test.js.snap
index b2371761b..63b967b9e 100644
--- a/tests/format/html/tags/__snapshots__/format.test.js.snap
+++ b/tests/format/html/tags/__snapshots__/format.test.js.snap
@@ -656,21 +656,27 @@ printWidth: 1
 <menu>
   <li>
     <button
-      onclick="copy()"
+      onclick="
+        copy()
+      "
     >
       Copy
     </button>
   </li>
   <li>
     <button
-      onclick="cut()"
+      onclick="
+        cut()
+      "
     >
       Cut
     </button>
   </li>
   <li>
     <button
-      onclick="paste()"
+      onclick="
+        paste()
+      "
     >
       Paste
     </button>
diff --git a/yarn.lock b/yarn.lock
index 775689a39..8e6b0fca6 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2296,6 +2296,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"@prettier/html-event-attributes@npm:0.0.1":
+  version: 0.0.1
+  resolution: "@prettier/html-event-attributes@npm:0.0.1"
+  checksum: 10/28c88092b06bf5a0ed2b9e8e4b7abf55eff409e64e0ebc958dd115f8c06c0667c2cb2df1bd4c55d28df58d481eadc7465e7138566b91288cafefc4b5ea048eb9
+  languageName: node
+  linkType: hard
+
 "@prettier/html-tags@npm:1.1.0":
   version: 1.1.0
   resolution: "@prettier/html-tags@npm:1.1.0"
@@ -7536,6 +7543,7 @@ __metadata:
     "@eslint/js": "npm:9.35.0"
     "@glimmer/syntax": "npm:0.95.0"
     "@prettier/cli": "npm:0.9.0"
+    "@prettier/html-event-attributes": "npm:0.0.1"
     "@prettier/html-tags": "npm:1.1.0"
     "@prettier/parse-srcset": "npm:3.1.0"
     "@stylistic/eslint-plugin": "npm:5.3.1"