fix(linter): allow () => undefined in noUselessUndefined
#8441
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🦋 Changeset detectedLatest commit: 3cfedab The changes in this PR will be included in the next version bump. This PR includes changesets to release 13 packages
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WalkthroughRemoves JsArrowFunctionExpression from the noUselessUndefined rule’s analysis and from the AnyUndefinedNode union, deletes related matching and mutation logic, and updates tests so Suggested reviewers
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🧹 Nitpick comments (1)
.changeset/fix-no-useless-undefined-arrow-function.md (1)
1-5: Nice and clear; consider adding a tiny code example. Per changeset guidelines, it’d be great to include e.g.const foo = () => undefined;as the concrete “now allowed” snippet.
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crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/invalid.json.snapis excluded by!**/*.snapand included by**crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/valid.json.snapis excluded by!**/*.snapand included by**
📒 Files selected for processing (4)
.changeset/fix-no-useless-undefined-arrow-function.md(1 hunks)crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs(1 hunks)crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/invalid.json(0 hunks)crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/valid.json(1 hunks)
💤 Files with no reviewable changes (1)
- crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/invalid.json
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📓 Path-based instructions (2)
.changeset/*.md
📄 CodeRabbit inference engine (CONTRIBUTING.md)
Write changesets that are concise (1-3 sentences), user-focused, use past tense for actions taken and present tense for Biome behavior, include code examples for rules, and end sentences with periods
Files:
.changeset/fix-no-useless-undefined-arrow-function.md
crates/**/*.rs
📄 CodeRabbit inference engine (CONTRIBUTING.md)
Update inline rustdoc documentation for rules, assists, and their options when adding new features or changing existing features in Rust crates
Files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
🧠 Learnings (22)
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUseless` prefix for rules that report unnecessary code that could be removed or simplified (e.g., `noUselessConstructor`)
Applied to files:
.changeset/fix-no-useless-undefined-arrow-function.mdcrates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUndeclared` prefix for rules that report undefined entities (e.g., `noUndeclaredVariables`)
Applied to files:
.changeset/fix-no-useless-undefined-arrow-function.mdcrates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noEmpty` prefix for rules that report empty code (e.g., `noEmptyBlockStatements`)
Applied to files:
.changeset/fix-no-useless-undefined-arrow-function.md
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUnused` prefix for rules that report unused entities (e.g., `noUnusedVariables`)
Applied to files:
.changeset/fix-no-useless-undefined-arrow-function.mdcrates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-21T01:10:53.059Z
Learnt from: dyc3
Repo: biomejs/biome PR: 8171
File: crates/biome_js_analyze/src/lint/nursery/no_leaked_render.rs:125-137
Timestamp: 2025-11-21T01:10:53.059Z
Learning: In the Biome codebase, each lint rule has its own options type declaration (e.g., `type Options = RuleNameOptions`) as part of the codegen process, even if the options struct is empty or unused. This is standard practice and should not be flagged as an issue.
Applied to files:
.changeset/fix-no-useless-undefined-arrow-function.md
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUnknown` prefix for rules that report mistyped entities in CSS (e.g., `noUnknownUnit`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUnsafe` prefix for rules that report code leading to runtime failures (e.g., `noUnsafeOptionalChaining`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noRedundant` prefix for rules that report redundant code (e.g., `noRedundantUseStrict`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Deprecate rules by adding a `deprecated` field to the `declare_lint_rule!` macro with a message explaining the reason for deprecation (e.g., 'Use the rule noAnotherVar')
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noInvalid` prefix for rules that report runtime errors from mistyping (e.g., `noInvalidConstructorSuper`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `useShorthand` prefix for rules that report syntax rewritable using equivalent compact syntax (e.g., `useShorthandAssign`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noRestricted` prefix for rules that report user-banned entities (e.g., `noRestrictedGlobals`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : The `action` function must return a `JsRuleAction` (or equivalent language-specific action type) with category `ctx.action_category(ctx.category(), ctx.group())` and applicability from `ctx.metadata().applicability()`
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use `Semantic<T>` query type instead of `Ast<T>` when a rule needs to access the semantic model for binding references and scope information
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Lint rules must be implemented using the `Rule` trait with type parameters: `Query` (node type to analyze), `State` (information for signals), `Signals` (return type from run function), and `Options` (rule configuration)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-24T18:05:27.810Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_js_formatter/CONTRIBUTING.md:0-0
Timestamp: 2025-11-24T18:05:27.810Z
Learning: Applies to crates/biome_js_formatter/**/*.rs : When formatting AST nodes, use mandatory tokens from the AST instead of hardcoding token strings (e.g., use `node.l_paren_token().format()` instead of `token("(")`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : The `diagnostic` function must return a `RuleDiagnostic` that defines the message reported to the user using the `markup!` macro
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Framework-specific rules should be named using the `use` or `no` prefix followed by the framework name (e.g., `noVueReservedProps`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Set the `language` field in `declare_lint_rule!` to the primary language (js for JavaScript, jsx for JSX, ts for TypeScript, tsx for TSX) that the rule applies to
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-24T18:05:27.810Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_js_formatter/CONTRIBUTING.md:0-0
Timestamp: 2025-11-24T18:05:27.810Z
Learning: Applies to crates/biome_js_formatter/**/*.rs : Import the `FormatNode` trait and implement it for your Node when creating formatters in biome_js_formatter
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/tests/specs/**/*.jsonc : Use `.jsonc` files in test specs with code snippets as array of strings to test rules in script environment (no import/export syntax)
Applied to files:
crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/valid.json
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/tests/specs/**/*.{js,ts,tsx,jsx,json,css} : Test rules using snapshot tests via the `insta` library with test cases in `tests/specs/<group>/<rule_name>/` directories prefixed by `invalid` or `valid`
Applied to files:
crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/valid.json
⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (12)
- GitHub Check: Lint project (depot-windows-2022)
- GitHub Check: Test (depot-ubuntu-24.04-arm-16)
- GitHub Check: Lint project (depot-ubuntu-24.04-arm-16)
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- GitHub Check: Test Node.js API
- GitHub Check: Bench (biome_js_parser)
- GitHub Check: Bench (biome_js_formatter)
- GitHub Check: Bench (biome_js_analyze)
- GitHub Check: autofix
🔇 Additional comments (3)
crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/valid.json (1)
3-5: Good regression coverage for the false positive. This test case being invalidmatches the new rule behaviour.crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs (2)
5-85: Behaviour change is clean (and removes the dodgy “fix”). DroppingJsArrowFunctionExpressionfromAnyUndefinedNodeis a straightforward way to stop the() => undefinedfalse positive and avoid the() => {}→noEmptyBlockStatementsping-pong.
101-211: No changes needed. TheNoUselessUndefinedOptionsstruct is empty with no configuration options (including nocheckArrowFunctionBody), so the rule correctly and unconditionally reports all uselessundefinedregardless of context. The current implementation matches the documented behaviour.
This change allows arrow function expressions with `undefined` body
to avoid conflicting with noEmptyBlockStatements rule.
Previously, `() => undefined` was flagged and the autofix suggested
`() => {}`, which would then trigger noEmptyBlockStatements.
Fixes biomejs#6577
40ce298 to
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🧹 Nitpick comments (1)
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs (1)
52-66: Consider adding() => undefinedto the Valid examples section.As per the coding guidelines, rule documentation should be updated when changing features. Since arrow functions with explicit
undefinedare now excluded, it would help users to see an explicit example in the Valid section, such as:function foo({bar}) {} +const foo = () => undefined; foo();This makes the exception immediately visible to users reading the rule documentation.
Based on coding guidelines requiring rustdoc updates for feature changes.
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⛔ Files ignored due to path filters (2)
crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/invalid.json.snapis excluded by!**/*.snapand included by**crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/valid.json.snapis excluded by!**/*.snapand included by**
📒 Files selected for processing (4)
.changeset/fix-no-useless-undefined-arrow-function.md(1 hunks)crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs(2 hunks)crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/invalid.json(0 hunks)crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/valid.json(1 hunks)
💤 Files with no reviewable changes (1)
- crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/invalid.json
🚧 Files skipped from review as they are similar to previous changes (2)
- .changeset/fix-no-useless-undefined-arrow-function.md
- crates/biome_js_analyze/tests/specs/nursery/noUselessUndefined/valid.json
🧰 Additional context used
📓 Path-based instructions (1)
crates/**/*.rs
📄 CodeRabbit inference engine (CONTRIBUTING.md)
Update inline rustdoc documentation for rules, assists, and their options when adding new features or changing existing features in Rust crates
Files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
🧠 Learnings (21)
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUndeclared` prefix for rules that report undefined entities (e.g., `noUndeclaredVariables`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUseless` prefix for rules that report unnecessary code that could be removed or simplified (e.g., `noUselessConstructor`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUnused` prefix for rules that report unused entities (e.g., `noUnusedVariables`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUnknown` prefix for rules that report mistyped entities in CSS (e.g., `noUnknownUnit`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noUnsafe` prefix for rules that report code leading to runtime failures (e.g., `noUnsafeOptionalChaining`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noEmpty` prefix for rules that report empty code (e.g., `noEmptyBlockStatements`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noRedundant` prefix for rules that report redundant code (e.g., `noRedundantUseStrict`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Deprecate rules by adding a `deprecated` field to the `declare_lint_rule!` macro with a message explaining the reason for deprecation (e.g., 'Use the rule noAnotherVar')
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noInvalid` prefix for rules that report runtime errors from mistyping (e.g., `noInvalidConstructorSuper`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `noRestricted` prefix for rules that report user-banned entities (e.g., `noRestrictedGlobals`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : The `action` function must return a `JsRuleAction` (or equivalent language-specific action type) with category `ctx.action_category(ctx.category(), ctx.group())` and applicability from `ctx.metadata().applicability()`
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use `Semantic<T>` query type instead of `Ast<T>` when a rule needs to access the semantic model for binding references and scope information
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Lint rules must be implemented using the `Rule` trait with type parameters: `Query` (node type to analyze), `State` (information for signals), `Signals` (return type from run function), and `Options` (rule configuration)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-24T18:05:27.810Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_js_formatter/CONTRIBUTING.md:0-0
Timestamp: 2025-11-24T18:05:27.810Z
Learning: Applies to crates/biome_js_formatter/**/*.rs : When formatting AST nodes, use mandatory tokens from the AST instead of hardcoding token strings (e.g., use `node.l_paren_token().format()` instead of `token("(")`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : The `diagnostic` function must return a `RuleDiagnostic` that defines the message reported to the user using the `markup!` macro
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Framework-specific rules should be named using the `use` or `no` prefix followed by the framework name (e.g., `noVueReservedProps`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `useShorthand` prefix for rules that report syntax rewritable using equivalent compact syntax (e.g., `useShorthandAssign`)
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Set the `language` field in `declare_lint_rule!` to the primary language (js for JavaScript, jsx for JSX, ts for TypeScript, tsx for TSX) that the rule applies to
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-24T18:05:27.810Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_js_formatter/CONTRIBUTING.md:0-0
Timestamp: 2025-11-24T18:05:27.810Z
Learning: Applies to crates/biome_js_formatter/**/*.rs : Import the `FormatNode` trait and implement it for your Node when creating formatters in biome_js_formatter
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-27T23:04:02.022Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_analyze/CONTRIBUTING.md:0-0
Timestamp: 2025-11-27T23:04:02.022Z
Learning: Applies to crates/biome_analyze/**/*analyze/src/lint/**/*.rs : Use the `declare_node_union!` macro to query multiple node types at once by joining them into an enum with `Any*Like` naming convention
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
📚 Learning: 2025-11-24T18:06:03.545Z
Learnt from: CR
Repo: biomejs/biome PR: 0
File: crates/biome_parser/CONTRIBUTING.md:0-0
Timestamp: 2025-11-24T18:06:03.545Z
Learning: Applies to crates/biome_parser/**/*.ungram : Unions of nodes must start with `Any*`, e.g., `AnyHtmlAttribute`
Applied to files:
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs
⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (12)
- GitHub Check: Lint project (depot-windows-2022)
- GitHub Check: Test (depot-ubuntu-24.04-arm-16)
- GitHub Check: Check Dependencies
- GitHub Check: Lint project (depot-ubuntu-24.04-arm-16)
- GitHub Check: End-to-end tests
- GitHub Check: Documentation
- GitHub Check: Test (depot-windows-2022-16)
- GitHub Check: Test Node.js API
- GitHub Check: autofix
- GitHub Check: Bench (biome_js_analyze)
- GitHub Check: Bench (biome_js_formatter)
- GitHub Check: Bench (biome_js_parser)
🔇 Additional comments (2)
crates/biome_js_analyze/src/lint/nursery/no_useless_undefined.rs (2)
6-8: LGTM!The removal of
JsArrowFunctionExpressionfrom imports is consistent with its exclusion from the rule's scope.
79-81: Excellent documentation of the exclusion.The inline comment clearly explains the rationale and references the issue—this helps future maintainers understand the decision.
CodSpeed Performance ReportMerging #8441 will not alter performanceComparing Summary
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Summary
Fixes #6577
This PR allows arrow function expressions with
undefinedbody (() => undefined) in thenoUselessUndefinedrule.Problem
Previously,
() => undefinedwas flagged as a useless undefined, and the autofix suggested replacing it with() => {}. However, this empty block would then trigger thenoEmptyBlockStatementsrule, creating a conflict between two rules.Solution
Per the discussion in the issue, the consensus (from @Conaclos) was to make an exception for
() => undefinedsince there's no good alternative:Changes
JsArrowFunctionExpressionhandling from the rule() => undefinedis no longer flagged() => { return undefined; }is still flagged (viaJsReturnStatementhandling)Test Plan
"const foo = () => undefined;"from invalid.json to valid.jsonThis PR was authored with the assistance of Claude Code.