.changeset/grumpy-monkeys-tap.md (1)

1-5: Add a tiny example to the changeset for clarity.

Guidelines recommend including a representative snippet. A minimal JS example helps users immediately see the effect.

Proposed addition:

 ---
 "@biomejs/biome": patch
 ---
 
 Fixed [#7301](https://github.com/biomejs/biome/issues/7301): [`useReadonlyClassProperties`](https://biomejs.dev/linter/rules/use-readonly-class-properties/) now correctly skips JavaScript files.
+#### Example.
+```js
+// JavaScript — no diagnostics from this rule.
+class C {
+  x = 1;
+  #y = 2;
+}
+```

crates/biome_js_analyze/src/lint/style/use_readonly_class_properties.rs (1)

275-281: Wording: “readonly decorator” → “readonly modifier”.

In TS this is a modifier, not a decorator. Minor, but user-facing.

-            markup! { "Add "<Emphasis>"readonly"</Emphasis>" decorator." }.to_owned(),
+            markup! { "Add "<Emphasis>"readonly"</Emphasis>" modifier." }.to_owned(),

crates/biome_js_analyze/tests/specs/style/useReadonlyClassProperties/valid.js (1)

1-7: Good JS coverage; consider a TS counterpart to prevent regressions.

Add a small invalid.ts or valid.ts to ensure the rule still runs on TS/TSX and produces/omits diagnostics as expected.

Example additions (as separate files):

// invalid.ts,expect_diagnostic
class C {
  private onlyInCtor = 1;
  constructor(x: number) { this.onlyInCtor = x; }
}

// valid.ts
class C {
  private readonly neverMutated = 1;
}

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Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-08-17T08:56:30.831Z
Learning: Applies to crates/biome_analyze/crates/**/tests/specs/**/{invalid*,valid*}.{js,jsx,ts,tsx,css,graphql,jsonc} : Place snapshot test cases under `tests/specs/<group>/<ruleName>/` using files prefixed with `invalid` and `valid`

Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-08-17T08:56:30.831Z
Learning: Applies to crates/biome_analyze/crates/biome_js_analyze/lib/src/lint/nursery/*.rs : For new JavaScript lint rules generated by `just new-js-lintrule`, implement the rule in the generated file under `biome_js_analyze/lib/src/lint/nursery/`

Learnt from: CR
PR: biomejs/biome#0
File: crates/biome_js_type_info/CONTRIBUTING.md:0-0
Timestamp: 2025-08-11T11:50:12.090Z
Learning: Applies to crates/biome_js_type_info/src/**/*.rs : Use TypeData::Unknown for unimplemented inference and TypeData::UnknownKeyword for the explicit TypeScript unknown keyword; treat them semantically the same but keep them distinct for measurement

crates/biome_js_analyze/src/lint/style/use_readonly_class_properties.rs (2)

13-14: Import of JsFileSource looks right.

This is the correct import to inspect source language in the rule context.

---

142-146: TSX is correctly treated as TypeScript by is_typescript()
The is_typescript() method matches any Language::TypeScript variant—including .tsx via JsFileSource::tsx()—so this rule will run on both .ts and .tsx files. No changes needed.