fix(lint/noUselessEscapeInString): flag useless escapes after skipping \${ in template literals (#6044)

Author: lucasweng

.changeset/petite-rats-sniff.md

@@ -0,0 +1,5 @@
+---
+"@biomejs/biome": patch
+---
+
+Fixed [#6042](https://github.com/biomejs/biome/pull/6042): [`noUselessEscapeInString`](https://next.biomejs.dev/linter/rules/no-useless-escape-in-string/) now reports useless escapes after skipping \${ in template literals.

crates/biome_js_analyze/src/lint/nursery/no_useless_escape_in_string.rs

@@ -165,6 +165,13 @@ fn next_useless_escape(str: &str, quote: u8) -> Option<usize> {
                     | b'u'
                     | b'v'
                     | b'x' => {}
+                    // In template literals, \${ is a valid escape for producing a literal ${
+                    b'$' => {
+                        // Clone iterator to peek ahead without advancing, so other escapes like \${\a aren't missed
+                        if !(quote == b'`' && (matches!(it.clone().next(), Some((_, b'{'))))) {
+                            return Some(i);
+                        }
+                    }
                     // Preserve escaping of Unicode characters U+2028 and U+2029
                     0xE2 => {
                         if !(matches!(it.next(), Some((_, 0x80)))
@@ -174,10 +181,6 @@ fn next_useless_escape(str: &str, quote: u8) -> Option<usize> {
                         }
                     }
                     _ => {
-                        // In template literals, \${ is a valid escape for producing a literal ${
-                        if quote == b'`' && c == b'$' && (matches!(it.next(), Some((_, b'{')))) {
-                            return None;
-                        }
                         // The quote can be escaped
                         if c != quote {
                             return Some(i);

crates/biome_js_analyze/tests/specs/nursery/noUselessEscapeInString/invalid.js

@@ -5,5 +5,7 @@ var s = {
     // A test with unicode characters that take more than one byte
     key: "😀\😀",
     // https://github.com/biomejs/biome/issues/6039
-    templateLiterals: `\$x`
+    templateLiterals1: `\$x`,
+    templateLiterals2: `\${\a`,
+    templateLiterals3: `\${} \a`
 };

crates/biome_js_analyze/tests/specs/nursery/noUselessEscapeInString/invalid.js.snap

@@ -12,7 +12,9 @@ var s = {
     // A test with unicode characters that take more than one byte
     key: "😀\😀",
     // https://github.com/biomejs/biome/issues/6039
-    templateLiterals: `\$x`
+    templateLiterals1: `\$x`,
+    templateLiterals2: `\${\a`,
+    templateLiterals3: `\${} \a`
 };
 
 ```
@@ -157,7 +159,7 @@ invalid.js:6:13 lint/nursery/noUselessEscapeInString  FIXABLE  ━━━━━
   > 6 │     key: "😀\😀",
       │              ^^
     7 │     // https://github.com/biomejs/biome/issues/6039
-    8 │     templateLiterals: `\$x`
+    8 │     templateLiterals1: `\$x`,
   
   i Only quotes that enclose the string and special characters need to be escaped.
   
@@ -169,22 +171,64 @@ invalid.js:6:13 lint/nursery/noUselessEscapeInString  FIXABLE  ━━━━━
 ```
 
 ```
-invalid.js:8:25 lint/nursery/noUselessEscapeInString  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+invalid.js:8:26 lint/nursery/noUselessEscapeInString  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
 
   ! The character doesn't need to be escaped.
   
      6 │     key: "😀\😀",
      7 │     // https://github.com/biomejs/biome/issues/6039
-   > 8 │     templateLiterals: `\$x`
-       │                         ^
-     9 │ };
-    10 │ 
+   > 8 │     templateLiterals1: `\$x`,
+       │                          ^
+     9 │     templateLiterals2: `\${\a`,
+    10 │     templateLiterals3: `\${} \a`
   
   i Only quotes that enclose the string and special characters need to be escaped.
   
   i Safe fix: Unescape the character.
   
-    8 │ ····templateLiterals:·`\$x`
-      │                        -   
+    8 │ ····templateLiterals1:·`\$x`,
+      │                         -    
+
+```
+
+```
+invalid.js:9:29 lint/nursery/noUselessEscapeInString  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  ! The character doesn't need to be escaped.
+  
+     7 │     // https://github.com/biomejs/biome/issues/6039
+     8 │     templateLiterals1: `\$x`,
+   > 9 │     templateLiterals2: `\${\a`,
+       │                             ^
+    10 │     templateLiterals3: `\${} \a`
+    11 │ };
+  
+  i Only quotes that enclose the string and special characters need to be escaped.
+  
+  i Safe fix: Unescape the character.
+  
+    9 │ ····templateLiterals2:·`\${\a`,
+      │                            -   
+
+```
+
+```
+invalid.js:10:31 lint/nursery/noUselessEscapeInString  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  ! The character doesn't need to be escaped.
+  
+     8 │     templateLiterals1: `\$x`,
+     9 │     templateLiterals2: `\${\a`,
+  > 10 │     templateLiterals3: `\${} \a`
+       │                               ^
+    11 │ };
+    12 │ 
+  
+  i Only quotes that enclose the string and special characters need to be escaped.
+  
+  i Safe fix: Unescape the character.
+  
+    10 │ ····templateLiterals3:·`\${}·\a`
+       │                              -  
 
 ```

crates/biome_js_analyze/tests/specs/nursery/noUselessEscapeInString/valid.js

@@ -3,5 +3,6 @@ var s = {
     '\0\'': "\n\"",
     "abc\u42efg": tagged` test ${1} \a`,
     key: `\``,
-    escapeTemplateLiteralInterpolation: `\${`
+    escapeTemplateLiteralInterpolation1: `\${`,
+    escapeTemplateLiteralInterpolation2: `\${}`
 };

crates/biome_js_analyze/tests/specs/nursery/noUselessEscapeInString/valid.js.snap

@@ -10,7 +10,8 @@ var s = {
     '\0\'': "\n\"",
     "abc\u42efg": tagged` test ${1} \a`,
     key: `\``,
-    escapeTemplateLiteralInterpolation: `\${`
+    escapeTemplateLiteralInterpolation1: `\${`,
+    escapeTemplateLiteralInterpolation2: `\${}`
 };
 
 ```