fix(parser): reject TypeScript class modifiers in JavaScript (#27312)

Reject TypeScript-only class member modifiers in JavaScript with TS8009. Valid JavaScript modifiers and members named `declare`, `override`, or `readonly` remain accepted.

Reuse the existing modifier validation and storage, retaining its deduplication and keyword-span behavior. Coverage fixtures and updated conformance snapshots cover ordinary, repeated, and escaped modifiers.

fixes #27290
closes #27294

crates/oxc_parser/src/diagnostics.rs

@@ -1197,6 +1197,11 @@ parser_diagnostics! {
         ts_error("5087", "A labeled tuple element is declared as rest with a '...' before the name, rather than before the type.").with_label(span)
     };
 
+    modifier_in_ts(kind: ModifierKind, span: Span) => {
+        ts_error("8009", format!("The '{kind}' modifier can only be used in TypeScript files."))
+            .with_label(span)
+    };
+
     parameter_modifiers_in_ts(modifier: Modifier, allowed: Option<ModifierKinds>) => {
         ts_error("8012", "Parameter modifiers can only be used in TypeScript files.")
             .with_label(modifier.span())

crates/oxc_parser/src/js/class.rs

@@ -267,6 +267,20 @@ impl<'a, C: Config> ParserImpl<'a, C> {
                 false,
                 diagnostics::cannot_appear_on_class_elements,
             );
+            if !parser.is_ts {
+                parser.verify_modifiers(
+                    modifiers,
+                    ModifierKinds::new([
+                        ModifierKind::Export,
+                        ModifierKind::Default,
+                        ModifierKind::Static,
+                        ModifierKind::Async,
+                        ModifierKind::Accessor,
+                    ]),
+                    false,
+                    |modifier, _| diagnostics::modifier_in_ts(modifier.kind, modifier.span()),
+                );
+            }
         }
 
         let start = self.cur_start();

tasks/coverage/misc/fail/oxc-27290-declare.js

@@ -0,0 +1,3 @@
+class A {
+  declare y;
+}

tasks/coverage/misc/fail/oxc-27290-escaped.js

@@ -0,0 +1,4 @@
+class A {
+  pr\u0069vate x;
+  publ\u0069c f() {}
+}

tasks/coverage/misc/fail/oxc-27290-private.js

@@ -0,0 +1,3 @@
+class A {
+  private g() {}
+}

tasks/coverage/misc/fail/oxc-27290-protected.js

@@ -0,0 +1,3 @@
+class A {
+  protected f() {}
+}

tasks/coverage/misc/fail/oxc-27290-readonly.js

@@ -0,0 +1,3 @@
+class A {
+  readonly x = 1;
+}

tasks/coverage/misc/fail/oxc-27290-repeated.js

@@ -0,0 +1,5 @@
+class A {
+  private private x = 1;
+  public public f() {}
+  readonly readonly y = 1;
+}

tasks/coverage/misc/pass/oxc-27290.js

@@ -0,0 +1,15 @@
+class A {
+  declare() {}
+  override() {}
+  readonly() {}
+
+  accessor value = 1;
+  static field = 1;
+  async method() {}
+}
+
+class B {
+  declare = 1;
+  override = 1;
+  readonly = 1;
+}

tasks/coverage/snapshots/codegen_misc.snap

@@ -1,3 +1,3 @@
 codegen_misc Summary:
-AST Parsed     : 112/112 (100.00%)
-Positive Passed: 112/112 (100.00%)
+AST Parsed     : 113/113 (100.00%)
+Positive Passed: 113/113 (100.00%)

tasks/coverage/snapshots/formatter_misc.snap

@@ -1,3 +1,3 @@
 formatter_misc Summary:
-AST Parsed     : 112/112 (100.00%)
-Positive Passed: 112/112 (100.00%)
+AST Parsed     : 113/113 (100.00%)
+Positive Passed: 113/113 (100.00%)

tasks/coverage/snapshots/lexer_misc.snap

@@ -1,3 +1,3 @@
 lexer_misc Summary:
-AST Parsed     : 353/353 (100.00%)
-Positive Passed: 353/353 (100.00%)
+AST Parsed     : 360/360 (100.00%)
+Positive Passed: 360/360 (100.00%)

tasks/coverage/snapshots/parser_misc.snap

@@ -1,7 +1,7 @@
 parser_misc Summary:
-AST Parsed     : 112/112 (100.00%)
-Positive Passed: 112/112 (100.00%)
-Negative Passed: 243/243 (100.00%)
+AST Parsed     : 113/113 (100.00%)
+Positive Passed: 113/113 (100.00%)
+Negative Passed: 249/249 (100.00%)
 
   × TS(1275): 'accessor' modifier can only appear on a property declaration.
    ╭─[misc/fail/accessor-modifier-abstract-method.ts:1:29]
@@ -4649,6 +4649,121 @@ Negative Passed: 243/243 (100.00%)
  7 │ }
    ╰────
 
+  × TS(8009): The 'declare' modifier can only be used in TypeScript files.
+   ╭─[misc/fail/oxc-27290-declare.js:2:3]
+ 1 │ class A {
+ 2 │   declare y;
+   ·   ───────
+ 3 │ }
+   ╰────
+
+  × Keywords cannot contain escape characters
+   ╭─[misc/fail/oxc-27290-escaped.js:2:3]
+ 1 │ class A {
+ 2 │   pr\u0069vate x;
+   ·   ────────────
+ 3 │   publ\u0069c f() {}
+   ╰────
+
+  × TS(8009): The 'private' modifier can only be used in TypeScript files.
+   ╭─[misc/fail/oxc-27290-escaped.js:2:3]
+ 1 │ class A {
+ 2 │   pr\u0069vate x;
+   ·   ───────
+ 3 │   publ\u0069c f() {}
+   ╰────
+
+  × Keywords cannot contain escape characters
+   ╭─[misc/fail/oxc-27290-escaped.js:3:3]
+ 2 │   pr\u0069vate x;
+ 3 │   publ\u0069c f() {}
+   ·   ───────────
+ 4 │ }
+   ╰────
+
+  × TS(8009): The 'public' modifier can only be used in TypeScript files.
+   ╭─[misc/fail/oxc-27290-escaped.js:3:3]
+ 2 │   pr\u0069vate x;
+ 3 │   publ\u0069c f() {}
+   ·   ──────
+ 4 │ }
+   ╰────
+
+  × TS(8009): The 'private' modifier can only be used in TypeScript files.
+   ╭─[misc/fail/oxc-27290-private.js:2:3]
+ 1 │ class A {
+ 2 │   private g() {}
+   ·   ───────
+ 3 │ }
+   ╰────
+
+  × TS(8009): The 'protected' modifier can only be used in TypeScript files.
+   ╭─[misc/fail/oxc-27290-protected.js:2:3]
+ 1 │ class A {
+ 2 │   protected f() {}
+   ·   ─────────
+ 3 │ }
+   ╰────
+
+  × TS(8009): The 'readonly' modifier can only be used in TypeScript files.
+   ╭─[misc/fail/oxc-27290-readonly.js:2:3]
+ 1 │ class A {
+ 2 │   readonly x = 1;
+   ·   ────────
+ 3 │ }
+   ╰────
+
+  × TS(1028): Accessibility modifier already seen.
+   ╭─[misc/fail/oxc-27290-repeated.js:2:11]
+ 1 │ class A {
+ 2 │   private private x = 1;
+   ·           ───────
+ 3 │   public public f() {}
+   ╰────
+  help: Remove the duplicate modifier.
+
+  × TS(8009): The 'private' modifier can only be used in TypeScript files.
+   ╭─[misc/fail/oxc-27290-repeated.js:2:11]
+ 1 │ class A {
+ 2 │   private private x = 1;
+   ·           ───────
+ 3 │   public public f() {}
+   ╰────
+
+  × TS(1028): Accessibility modifier already seen.
+   ╭─[misc/fail/oxc-27290-repeated.js:3:10]
+ 2 │   private private x = 1;
+ 3 │   public public f() {}
+   ·          ──────
+ 4 │   readonly readonly y = 1;
+   ╰────
+  help: Remove the duplicate modifier.
+
+  × TS(8009): The 'public' modifier can only be used in TypeScript files.
+   ╭─[misc/fail/oxc-27290-repeated.js:3:10]
+ 2 │   private private x = 1;
+ 3 │   public public f() {}
+   ·          ──────
+ 4 │   readonly readonly y = 1;
+   ╰────
+
+  × TS(1030): 'readonly' modifier already seen.
+   ╭─[misc/fail/oxc-27290-repeated.js:4:12]
+ 3 │   public public f() {}
+ 4 │   readonly readonly y = 1;
+   ·            ────────
+ 5 │ }
+   ╰────
+  help: Remove the duplicate modifier.
+
+  × TS(8009): The 'readonly' modifier can only be used in TypeScript files.
+   ╭─[misc/fail/oxc-27290-repeated.js:4:12]
+ 3 │   public public f() {}
+ 4 │   readonly readonly y = 1;
+   ·            ────────
+ 5 │ }
+   ╰────
+
   × Expected `:` but found `[`
    ╭─[misc/fail/oxc-3320.tsx:1:8]
  1 │ m< $<{3[   $<{3[  $<{3[ m< m$<{3[ m< mm< $<{3[   $<{3[  $<{3[ m< m$<{3[ m< m$<{3[  $<{3[ m< m$<{3[

tasks/coverage/snapshots/parser_typescript.snap

@@ -3,7 +3,7 @@ commit: f29aeb9f
 parser_typescript Summary:
 AST Parsed     : 9658/9666 (99.92%)
 Positive Passed: 9651/9666 (99.84%)
-Negative Passed: 1689/2759 (61.22%)
+Negative Passed: 1691/2759 (61.29%)
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/FunctionDeclaration3.ts
 
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/FunctionDeclaration4.ts
@@ -570,8 +570,6 @@ Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler
 
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/jsFileCompilationOptionalClassElementSyntaxOfClass.ts
 
-Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/jsFileCompilationPublicMethodSyntaxOfClass.ts
-
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/jsFunctionWithPrototypeNoErrorTruncationNoCrash.ts
 
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/jsTypedefMergedWithModuleExportProperty.ts
@@ -1802,8 +1800,6 @@ Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/conforma
 
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/node/nodeModulesTopLevelAwait.ts
 
-Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/override/override_js3.ts
-
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/parser/ecmascript3/Accessors/parserES3Accessors1.ts
 
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/parser/ecmascript3/Accessors/parserES3Accessors3.ts
@@ -2764,6 +2760,14 @@ Expect to Parse: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/
  7 │ }
    ╰────
 
+  × TS(8009): The 'declare' modifier can only be used in TypeScript files.
+   ╭─[typescript/tsc/testdata/tests/cases/compiler/ambientPropertyDeclarationInJs.ts:6:5]
+ 5 │ 
+ 6 │     declare prop: string;
+   ·     ───────
+ 7 │ 
+   ╰────
+
   × Expected `;` but found `:`
    ╭─[typescript/tsc/testdata/tests/cases/compiler/ambientPropertyDeclarationInJs.ts:6:17]
  5 │ 
@@ -8933,6 +8937,14 @@ Expect to Parse: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/
    ·           ╰── Opened here
    ╰────
 
+  × TS(8009): The 'public' modifier can only be used in TypeScript files.
+   ╭─[typescript/tsc/testdata/tests/cases/compiler/jsFileCompilationPublicMethodSyntaxOfClass.ts:2:5]
+ 1 │ class C {
+ 2 │     public foo() {
+   ·     ──────
+ 3 │     }
+   ╰────
+
   × TS(8012): Parameter modifiers can only be used in TypeScript files.
    ╭─[typescript/tsc/testdata/tests/cases/compiler/jsFileCompilationPublicParameterModifier.ts:1:23]
  1 │ class C { constructor(public x) { }}
@@ -25981,6 +25993,14 @@ Expect to Parse: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/
     ╰────
   help: No modifiers are allowed here.
 
+  × TS(8009): The 'override' modifier can only be used in TypeScript files.
+   ╭─[typescript/tsc/testdata/tests/cases/conformance/override/override_js3.ts:7:5]
+ 6 │ class D extends B {
+ 7 │     override foo (v) {}
+   ·     ────────
+ 8 │     /** @override */
+   ╰────
+
   × Cannot use `await` as an identifier in an async context
    ╭─[typescript/tsc/testdata/tests/cases/conformance/parser/ecmascript2018/asyncGenerators/parser.asyncGenerators.classMethods.es2018.ts:3:15]
  2 │ class C4 {

tasks/coverage/snapshots/semantic_misc.snap

@@ -1,6 +1,6 @@
 semantic_misc Summary:
-AST Parsed     : 112/112 (100.00%)
-Positive Passed: 106/112 (94.64%)
+AST Parsed     : 113/113 (100.00%)
+Positive Passed: 107/113 (94.69%)
 semantic Error: tasks/coverage/misc/pass/declare-let-private.ts
 Bindings mismatch:
 after transform: ScopeId(0): ["private"]

tasks/coverage/snapshots/transformer_misc.snap

@@ -1,3 +1,3 @@
 transformer_misc Summary:
-AST Parsed     : 112/112 (100.00%)
-Positive Passed: 112/112 (100.00%)
+AST Parsed     : 113/113 (100.00%)
+Positive Passed: 113/113 (100.00%)