Don't remove uselsess \ in string literals (#16563)

changelog_unreleased/javascript/16563.md

@@ -0,0 +1,23 @@
+#### Don't remove useless `\` in string literals (#16563 by @sosukesuzuki)
+
+Previously, Prettier would remove useless escape characters (`\`) from string literals. However, this behavior was inconsistent as it did not apply to template literals, which was reported in [issue #16542](https://github.com/prettier/prettier/issues/16542).
+
+This issue is a feature request to extend this behavior to template literals as well.
+
+After discussing this internally, the Prettier team concluded that removing useless escape characters, whether in string literals or template literals, is the responsibility of a linter, not a formatter.
+
+This change disables the removal of useless escape characters in string literals across all languages supported by Prettier. To keep the previous behavior, we recommend using ESLint rules like [`no-useless-escape`](https://eslint.org/docs/latest/rules/no-useless-escape).
+
+Escaping quotes within string literals (e.g. `"\"\'"`) works the same as before. This is explained on the [Rationale](https://prettier.io/docs/en/rationale.html#strings) page of the official documentation.
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+const str = "\a";
+
+// Prettier stable
+const str = "a";
+
+// Prettier main
+const str = "\a";
+```

src/utils/print-string.js

@@ -33,12 +33,10 @@ function printString(raw, options) {
   return makeString(
     rawContent,
     enclosingQuote,
-    !(
+    // Until Prettier 3.3.3, this option was set to true for most parsers, with some exceptions like CSS.
-      options.parser === "css" ||
+    // Since Prettier 3.3.4, it is set to false for all parsers.
-      options.parser === "less" ||
+    // For more details, please see https://github.com/prettier/prettier/issues/16542#issuecomment-2282249280.
-      options.parser === "scss" ||
+    false,
-      options.__embeddedInHtml
-    ),
   );
 }
 

tests/format/flow-repo/malformed_code/__snapshots__/format.test.js.snap

@@ -17,7 +17,7 @@ const error: string = 123; // Should be an error, but can't lex this file
 // @flow
 // Example found at
 // https://github.com/sebmarkbage/art/blob/51ffce8164a555d652843241c2fdda52e186cbbd/parsers/svg/text.js#L137
-const evil_regex_as_a_string = "/[s�]*$/";
+const evil_regex_as_a_string = "/[\\s�]*$/";
 
 const error: string = 123; // Should be an error, but can't lex this file
 

tests/format/js/quote-props/__snapshots__/format.test.js.snap

@@ -496,7 +496,7 @@ Object.entries({
 };
 
 a = {
-  a: 1,
+  "\\a": 1,
   b: 2,
 };
 
@@ -714,8 +714,8 @@ Object.entries({
 };
 
 a = {
-  a: 1,
+  '\\a': 1,
-  b: 2,
+  'b': 2,
 };
 
 ================================================================================
@@ -931,8 +931,8 @@ Object.entries({
 };
 
 a = {
-  a: 1,
+  "\\a": 1,
-  b: 2,
+  "b": 2,
 };
 
 ================================================================================
@@ -1148,7 +1148,7 @@ Object.entries({
 };
 
 a = {
-  "a": 1,
+  "\\a": 1,
   "b": 2,
 };
 

tests/format/js/quotes/__snapshots__/format.test.js.snap

@@ -236,21 +236,21 @@ singleQuote: true
 // Unnecessary escapes.
 ("'");
 ('"');
-('a');
+('\\a');
-('a');
+('\\a');
-('hola');
+('hol\\a');
-('hola');
+('hol\\a');
 ('hol\\\\a (the a is not escaped)');
 ('hol\\\\a (the a is not escaped)');
-('multiple a unnecessary a escapes');
+('multiple \\a unnecessary \\a escapes');
-('multiple a unnecessary a escapes');
+('multiple \\a unnecessary \\a escapes');
-('unnecessarily escaped character preceded by escaped backslash \\\\a');
+('unnecessarily escaped character preceded by escaped backslash \\\\\\a');
-('unnecessarily escaped character preceded by escaped backslash \\\\a');
+('unnecessarily escaped character preceded by escaped backslash \\\\\\a');
 ('unescaped character preceded by two escaped backslashes       \\\\\\\\a');
 ('unescaped character preceded by two escaped backslashes       \\\\\\\\a');
-('aa'); // consecutive unnecessarily escaped characters
+('\\a\\a'); // consecutive unnecessarily escaped characters
-('aa'); // consecutive unnecessarily escaped characters
+('\\a\\a'); // consecutive unnecessarily escaped characters
-('escaped \\u2030 ‰ (should not stay escaped)');
+('escaped \\u2030 \\‰ (should not stay escaped)');
 
 // Meaningful escapes
 ('octal escapes \\0 \\1 \\2 \\3 \\4 \\5 \\6 \\7');
@@ -448,21 +448,21 @@ printWidth: 80
 // Unnecessary escapes.
 ("'");
 ('"');
-("a");
+("\\a");
-("a");
+("\\a");
-("hola");
+("hol\\a");
-("hola");
+("hol\\a");
 ("hol\\\\a (the a is not escaped)");
 ("hol\\\\a (the a is not escaped)");
-("multiple a unnecessary a escapes");
+("multiple \\a unnecessary \\a escapes");
-("multiple a unnecessary a escapes");
+("multiple \\a unnecessary \\a escapes");
-("unnecessarily escaped character preceded by escaped backslash \\\\a");
+("unnecessarily escaped character preceded by escaped backslash \\\\\\a");
-("unnecessarily escaped character preceded by escaped backslash \\\\a");
+("unnecessarily escaped character preceded by escaped backslash \\\\\\a");
 ("unescaped character preceded by two escaped backslashes       \\\\\\\\a");
 ("unescaped character preceded by two escaped backslashes       \\\\\\\\a");
-("aa"); // consecutive unnecessarily escaped characters
+("\\a\\a"); // consecutive unnecessarily escaped characters
-("aa"); // consecutive unnecessarily escaped characters
+("\\a\\a"); // consecutive unnecessarily escaped characters
-("escaped \\u2030 ‰ (should not stay escaped)");
+("escaped \\u2030 \\‰ (should not stay escaped)");
 
 // Meaningful escapes
 ("octal escapes \\0 \\1 \\2 \\3 \\4 \\5 \\6 \\7");

tests/format/js/strings/__snapshots__/format.test.js.snap

@@ -154,10 +154,10 @@ trailingComma: "all"
 =====================================output=====================================
 // https://github.com/babel/babel/pull/11852
 
-"8", "9";
+"\\8", "\\9";
 () => {
   "use strict";
-  "8", "9";
+  "\\8", "\\9";
 };
 
 ================================================================================
@@ -205,10 +205,10 @@ trailingComma: "es5"
 =====================================output=====================================
 // https://github.com/babel/babel/pull/11852
 
-"8", "9";
+"\\8", "\\9";
 () => {
   "use strict";
-  "8", "9";
+  "\\8", "\\9";
 };
 
 ================================================================================

tests/format/json/json-superset/format.test.js

@@ -25,7 +25,7 @@ runFormatTest(
         code: SINGLE_QUOTE + BACKSLASH + character + SINGLE_QUOTE,
         output:
           character === UNDERSCORE || character === SPACE
-            ? DOUBLE_QUOTE + character + DOUBLE_QUOTE + LINE_FEED
+            ? DOUBLE_QUOTE + BACKSLASH + character + DOUBLE_QUOTE + LINE_FEED
             : DOUBLE_QUOTE + BACKSLASH + character + DOUBLE_QUOTE + LINE_FEED,
       })),
       ...characters.map((character) => ({

tests/format/json/json/__snapshots__/format.test.js.snap

@@ -1536,7 +1536,7 @@ trailingComma: "all"
     "quote": "\\"",
     "backslash": "\\\\",
     "controls": "\\b\\f\\n\\r\\t",
-    "slash": "/ & /",
+    "slash": "/ & \\/",
     "alpha": "abcdefghijklmnopqrstuvwyz",
     "ALPHA": "ABCDEFGHIJKLMNOPQRSTUVWYZ",
     "digit": "0123456789",
@@ -1560,7 +1560,7 @@ trailingComma: "all"
     "compact": [1, 2, 3, 4, 5, 6, 7],
     "jsontext": "{\\"object with 1 member\\":[\\"array with 1 element\\"]}",
     "quotes": "" \\u0022 %22 0x22 034 "",
-    "/\\\\\\"\\uCAFE\\uBABE\\uAB98\\uFCDE\\ubcda\\uef4A\\b\\f\\n\\r\\t\`1~!@#$%^&*()_+-=[]{}|;:',./<>?": "A key can be any string"
+    "\\/\\\\\\"\\uCAFE\\uBABE\\uAB98\\uFCDE\\ubcda\\uef4A\\b\\f\\n\\r\\t\`1~!@#$%^&*()_+-=[]{}|;:',./<>?": "A key can be any string"
   },
   0.5,
   98.6,
@@ -1667,7 +1667,7 @@ trailingComma: "all"
     quote: '"',
     backslash: "\\\\",
     controls: "\\b\\f\\n\\r\\t",
-    slash: "/ & /",
+    slash: "/ & \\/",
     alpha: "abcdefghijklmnopqrstuvwyz",
     ALPHA: "ABCDEFGHIJKLMNOPQRSTUVWYZ",
     digit: "0123456789",
@@ -1691,7 +1691,7 @@ trailingComma: "all"
     compact: [1, 2, 3, 4, 5, 6, 7],
     jsontext: '{"object with 1 member":["array with 1 element"]}',
     quotes: "&#34; \\u0022 %22 0x22 034 &#x22;",
-    "/\\\\\\"\\uCAFE\\uBABE\\uAB98\\uFCDE\\ubcda\\uef4A\\b\\f\\n\\r\\t\`1~!@#$%^&*()_+-=[]{}|;:',./<>?": "A key can be any string",
+    "\\/\\\\\\"\\uCAFE\\uBABE\\uAB98\\uFCDE\\ubcda\\uef4A\\b\\f\\n\\r\\t\`1~!@#$%^&*()_+-=[]{}|;:',./<>?": "A key can be any string",
   },
   0.5,
   98.6,
@@ -1798,7 +1798,7 @@ trailingComma: "es5"
     "quote": "\\"",
     "backslash": "\\\\",
     "controls": "\\b\\f\\n\\r\\t",
-    "slash": "/ & /",
+    "slash": "/ & \\/",
     "alpha": "abcdefghijklmnopqrstuvwyz",
     "ALPHA": "ABCDEFGHIJKLMNOPQRSTUVWYZ",
     "digit": "0123456789",
@@ -1822,7 +1822,7 @@ trailingComma: "es5"
     "compact": [1, 2, 3, 4, 5, 6, 7],
     "jsontext": "{\\"object with 1 member\\":[\\"array with 1 element\\"]}",
     "quotes": "&#34; \\u0022 %22 0x22 034 &#x22;",
-    "/\\\\\\"\\uCAFE\\uBABE\\uAB98\\uFCDE\\ubcda\\uef4A\\b\\f\\n\\r\\t\`1~!@#$%^&*()_+-=[]{}|;:',./<>?": "A key can be any string"
+    "\\/\\\\\\"\\uCAFE\\uBABE\\uAB98\\uFCDE\\ubcda\\uef4A\\b\\f\\n\\r\\t\`1~!@#$%^&*()_+-=[]{}|;:',./<>?": "A key can be any string"
   },
   0.5,
   98.6,
@@ -1929,7 +1929,7 @@ trailingComma: "es5"
     quote: '"',
     backslash: "\\\\",
     controls: "\\b\\f\\n\\r\\t",
-    slash: "/ & /",
+    slash: "/ & \\/",
     alpha: "abcdefghijklmnopqrstuvwyz",
     ALPHA: "ABCDEFGHIJKLMNOPQRSTUVWYZ",
     digit: "0123456789",
@@ -1953,7 +1953,7 @@ trailingComma: "es5"
     compact: [1, 2, 3, 4, 5, 6, 7],
     jsontext: '{"object with 1 member":["array with 1 element"]}',
     quotes: "&#34; \\u0022 %22 0x22 034 &#x22;",
-    "/\\\\\\"\\uCAFE\\uBABE\\uAB98\\uFCDE\\ubcda\\uef4A\\b\\f\\n\\r\\t\`1~!@#$%^&*()_+-=[]{}|;:',./<>?": "A key can be any string",
+    "\\/\\\\\\"\\uCAFE\\uBABE\\uAB98\\uFCDE\\ubcda\\uef4A\\b\\f\\n\\r\\t\`1~!@#$%^&*()_+-=[]{}|;:',./<>?": "A key can be any string",
   },
   0.5,
   98.6,