feat(lint/useIterableCallbackReturn): add allowImplicit option by mvanhorn · Pull Request #9489 · biomejs/biome

mvanhorn · 2026-03-15T15:38:40Z

Summary

Adds an allowImplicit option to the useIterableCallbackReturn rule, matching ESLint's allowImplicit option for array-callback-return.

When allowImplicit is true, callbacks for methods that require a return value (like map, filter, find, etc.) can use return; to implicitly return undefined. This supports patterns like:

[1, 2, 3].map((x) => {
    if (x > 2) return x;
    return; // intentionally returns undefined
});

The default is false, preserving the existing behavior.

As discussed in the issue, this was identified as an enhancement rather than a bug fix. The maintainer confirmed this should match ESLint's behavior.

Fixes #9445

Test Plan

cargo check -p biome_js_analyze passes
Rule documentation updated with option description and examples

Docs

Rule documentation is updated inline with the new allowImplicit option, including a JSON config example and a JavaScript code example.

AI Assistance

This PR was written with AI assistance (Claude Code).

Add `allowImplicit` option that permits callbacks of methods requiring return values (map, filter, etc.) to use `return;` for implicit undefined returns. This matches ESLint's `allowImplicit` option for the `array-callback-return` rule. When enabled, `return;` in map/filter/etc. callbacks is accepted as intentional. When disabled (the default), the existing behavior is preserved. Fixes biomejs#9445 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

changeset-bot · 2026-03-15T15:38:48Z

🦋 Changeset detected

Latest commit: 6c22bb1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 14 packages

Name	Type
@biomejs/biome	Minor
@biomejs/cli-win32-x64	Minor
@biomejs/cli-win32-arm64	Minor
@biomejs/cli-darwin-x64	Minor
@biomejs/cli-darwin-arm64	Minor
@biomejs/cli-linux-x64	Minor
@biomejs/cli-linux-arm64	Minor
@biomejs/cli-linux-x64-musl	Minor
@biomejs/cli-linux-arm64-musl	Minor
@biomejs/wasm-web	Minor
@biomejs/wasm-bundler	Minor
@biomejs/wasm-nodejs	Minor
@biomejs/backend-jsonrpc	Patch
@biomejs/js-api	Major

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coderabbitai · 2026-03-15T15:45:37Z

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Walkthrough

This PR introduces an allowImplicit option to the useIterableCallbackReturn lint rule. When enabled, the rule permits explicit bare return statements (returning undefined) within iterable callbacks. The changes span the rule implementation, options configuration, and test cases. When allowImplicit is true, the diagnostic logic branches differently, allowing implicit undefined returns whilst still detecting genuinely suspicious return patterns. Default behaviour remains unchanged.

Possibly related PRs

feat: add the checkForEach option to useIterableCallbackReturn #8289: Modifies the same lint rule and its options handling infrastructure in parallel.
fix(lint/suspicious/useIterableCallbackReturn): single-expression void arrow function #7564: Modifies the same rule's return-path classification and diagnostic generation logic.

Suggested labels

A-Diagnostic

Suggested reviewers

ematipico
dyc3

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarises the main change: adding an allowImplicit option to the useIterableCallbackReturn rule.
Description check	✅ Passed	The description clearly explains the allowImplicit option, its purpose, examples, and how it aligns with ESLint's behaviour.
Linked Issues check	✅ Passed	The PR fully addresses issue `#9445` by implementing the allowImplicit option to permit bare returns in iterable callbacks whilst preserving existing detection of truly suspicious returns.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing the allowImplicit option and its test coverage; no unrelated modifications detected.

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coderabbitai

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@crates/biome_js_analyze/src/lint/suspicious/use_iterable_callback_return.rs`:
- Around line 217-233: The allow_implicit branch currently only reports
MissingReturnWithValue when there are exclusively fall-through paths and no
returns at all, missing cases where some explicit returns exist but other paths
fall through; update the block guarded by allow_implicit to mirror the logic
used in the non-allow branch for fall-throughs: if
returns_info.has_paths_without_returns then if
returns_info.returns_with_value.is_empty() push
RuleProblemKind::MissingReturnWithValue else push
RuleProblemKind::NotAllPathsReturnValue, leaving the treatment of
returns_without_value unchanged; adjust the code around allow_implicit,
returns_info, and problems to implement this behavior.

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crates/biome_js_analyze/src/lint/suspicious/use_iterable_callback_return.rs
crates/biome_rule_options/src/use_iterable_callback_return.rs

coderabbitai

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In @.changeset/warm-clouds-float.md:
- Around line 1-3: The changeset currently declares a patch release for the
package "@biomejs/biome" but adds a new option (a feature) so update the
changeset header in .changeset/warm-clouds-float.md: change the value from
"patch" to "minor" for the "@biomejs/biome" entry so the release type correctly
reflects a minor (feature) bump.

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.changeset/warm-clouds-float.md

Adding a new option is a feature addition under semver. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…cit mode When allowImplicit is true and some paths return values while others fall through without any return, report NotAllPathsReturnValue. Previously only the case where NO returns existed at all was caught. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

coderabbitai

🧹 Nitpick comments (1)

crates/biome_js_analyze/tests/specs/suspicious/useIterableCallbackReturn/allowImplicit.js (1)
1-31: Good test coverage for the core allowImplicit behaviour.

The test cases comprehensively cover the main scenarios. One small suggestion: the PR objectives mention the .filter(Boolean) chaining pattern as a use case. You might consider adding a test demonstrating this idiom:
// Valid: mapping with intentional undefined, then filtering
[1, 2, 3].map((x) => {
    if (x > 2) return x;
    return;
}).filter(Boolean);
That said, functionally it's the same as case 1, so this is entirely optional.

,
🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed.

In
`@crates/biome_js_analyze/tests/specs/suspicious/useIterableCallbackReturn/allowImplicit.js`
around lines 1 - 31, Add a test case to the existing allowImplicit spec
demonstrating the common `.map(...).filter(Boolean)` idiom: append a snippet
that maps with an explicit empty `return;` on some paths (same pattern as the
first valid case) and then chains `.filter(Boolean)` to show that intentional
undefineds are filtered out; reference the arrow callback used in the existing
tests (the `.map((x) => { if (x > 2) return x; return; })` pattern) so the new
test mirrors that behavior but adds the `.filter(Boolean)` call to the
expression.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In
`@crates/biome_js_analyze/tests/specs/suspicious/useIterableCallbackReturn/allowImplicit.js`:
- Around line 1-31: Add a test case to the existing allowImplicit spec
demonstrating the common `.map(...).filter(Boolean)` idiom: append a snippet
that maps with an explicit empty `return;` on some paths (same pattern as the
first valid case) and then chains `.filter(Boolean)` to show that intentional
undefineds are filtered out; reference the arrow callback used in the existing
tests (the `.map((x) => { if (x > 2) return x; return; })` pattern) so the new
test mirrors that behavior but adds the `.filter(Boolean)` call to the
expression.

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📒 Files selected for processing (3)

crates/biome_js_analyze/src/lint/suspicious/use_iterable_callback_return.rs
crates/biome_js_analyze/tests/specs/suspicious/useIterableCallbackReturn/allowImplicit.js
crates/biome_js_analyze/tests/specs/suspicious/useIterableCallbackReturn/allowImplicit.options.json

ematipico

Thank you! Still some work required

tests needs to be separated
the PR most go next, as per contribution guidelines

ematipico · 2026-03-16T06:50:34Z

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+    /// This is useful for patterns like mapping with early returns where some paths
+    /// intentionally return `undefined`.
+    ///
+    /// This matches ESLint's [`allowImplicit`](https://eslint.org/docs/latest/rules/array-callback-return#allowimplicit) option.


This information isn't needed

ematipico · 2026-03-16T06:53:48Z

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+    /// [1, 2, 3].map((x) => {
+    ///     if (x > 2) return x;
+    ///     return;
+    /// });


Can we have a snippet that is more real?

For example we should a .filter(Boolean)

ematipico · 2026-03-16T06:54:50Z

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@@ -0,0 +1,31 @@
+// Valid: empty return is accepted with allowImplicit


The file should have // should not generate diagnostics comment on top

ematipico · 2026-03-16T06:55:54Z

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+[1, 2, 3].map((x) => {
+    if (x > 2) return x;
+    return;
+});
+
+// Valid: all paths return values
+[1, 2, 3].map((x) => {
+    if (x > 2) return x;
+    return 0;
+});
+
+// Valid: implicit return (arrow expression)
+[1, 2, 3].map((x) => x * 2);
+
+// Invalid: some paths return values, others fall through without any return
+[1, 2, 3].map((x) => {
+    if (x > 2) return x;
+    // falls through - likely a bug
+});
+
+// Invalid: no returns at all
+[1, 2, 3].map((x) => {
+    console.log(x);
+});
+
+// Valid: only empty returns (all paths covered)
+[1, 2, 3].forEach((x) => {
+    if (x > 2) return;
+    console.log(x);
+});


We don't mix valid and invalid cases. Valid in a file, invalid in another file

- Split allowImplicit tests into valid/invalid files per convention - Add "// should not generate diagnostics" header to valid file - Replace basic map example with .filter(Boolean) pattern in docs - Remove ESLint reference and inline comments from rule logic

ematipico · 2026-03-16T15:07:44Z

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+    /// }).filter(Boolean);
+    /// ```
+    ///
+    /// When `allowImplicit` is `true`, the above code will not trigger any diagnostic.


Suggested change

/// When `allowImplicit` is `true`, the above code will not trigger any diagnostic.

That's implied by the infrastructure

ematipico · 2026-03-16T15:08:54Z

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@@ -0,0 +1,10 @@
+// some paths return values, others fall through without any return


Invalid files must have // should not generate diagnostics. You're using an AI agent, load the skills we provide.

Yeah sorry, between the two files I got confused, but it's easily understandable:

valid -> should not

invalid -> should

Every file should have the top-level comments

ematipico · 2026-03-16T15:09:08Z

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@@ -0,0 +1,29 @@
+// should not generate diagnostics


That's wrong

- Remove "When allowImplicit is true..." line (implied by infrastructure) - Fix valid test file header: use /* */ block comment to match convention - Verified all 10 useIterableCallbackReturn tests pass locally

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

codspeed-hq · 2026-04-16T14:45:27Z

Merging this PR will not alter performance

✅ 58 untouched benchmarks
⏩ 196 skipped benchmarks¹

_{Comparing mvanhorn:osc/9445-allow-implicit-callback-return (6c22bb1) with next (aaf4af0)}

196 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

ematipico · 2026-05-19T21:00:46Z

Yet another bot

github-actions Bot added A-Linter Area: linter L-JavaScript Language: JavaScript and super languages labels Mar 15, 2026

coderabbitai Bot reviewed Mar 15, 2026

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Comment thread crates/biome_js_analyze/src/lint/suspicious/use_iterable_callback_return.rs

chore: add changeset for biomejs#9445

78b550c

coderabbitai Bot reviewed Mar 15, 2026

View reviewed changes

Comment thread .changeset/warm-clouds-float.md

Matt Van Horn and others added 2 commits March 15, 2026 10:13

chore: bump changeset from patch to minor

e89b399

Adding a new option is a feature addition under semver. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

coderabbitai Bot reviewed Mar 15, 2026

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ematipico requested changes Mar 16, 2026

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Matt Van Horn and others added 2 commits March 16, 2026 08:39

Address review: remove implied doc line, fix valid test header format

629f724

- Remove "When allowImplicit is true..." line (implied by infrastructure) - Fix valid test file header: use /* */ block comment to match convention - Verified all 10 useIterableCallbackReturn tests pass locally

fix(test): add missing diagnostics header to allowImplicitInvalid.js

fa7c864

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

daniele-orlando mentioned this pull request Apr 16, 2026

💅 lint/suspicious/useIterableCallbackReturn makes impossible to explicitly return undefined #9445

Closed

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ematipico and others added 2 commits April 16, 2026 15:19

Merge branch 'next' into osc/9445-allow-implicit-callback-return

3085183

[autofix.ci] apply automated fixes

f9327c2

Merge branch 'next' into osc/9445-allow-implicit-callback-return

6c22bb1

ematipico closed this May 19, 2026

ematipico added the M-Likely Agent Meta: this was likely an automated PR without a human in the loop label May 19, 2026

TXWSLYF mentioned this pull request May 25, 2026

feat(lint/useIterableCallbackReturn): add allowImplicit option #9445 #10461

Merged

		@@ -0,0 +1,31 @@
		// Valid: empty return is accepted with allowImplicit

		@@ -0,0 +1,10 @@
		// some paths return values, others fall through without any return

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mvanhorn commented Mar 15, 2026

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Merging this PR will not alter performance

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