fix: only flag object assign in accumulators, if it is allocating a new object for each iter #7078

emilyinure · 2025-07-31T18:49:39Z

… object on every assignment

Summary

Accumulating Object.Assign isn't inherently bad as not all calls create new objects which result in the O(n*n) memory usage.

#6675
#5277

Test Plan

Testing shows that only when we are creating a new object every iteration, and assigning to it from the accumulator, it gets flagged

… object on every assignment biomejs#6675

…ing new keyword to allocate target objects

changeset-bot · 2025-07-31T18:49:44Z

🦋 Changeset detected

Latest commit: 984ea89

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

This PR includes changesets to release 13 packages

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

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codspeed-hq · 2025-07-31T19:03:32Z

CodSpeed Performance Report

Merging #7078 will not alter performance

_{Comparing emilyinure:fix-accumulating-object-creation (984ea89) with main (0c8349e)}

Summary

✅ 58 untouched
⏩ 95 skipped¹

95 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 · 2025-08-01T09:36:05Z

Thank you @emilyinure, there are a couple of things to address:

Create a changeset. Remember to write the changeset for end-users, so they can understand the fix.
It seems that the examples of the documentation aren't valid anymore and need to be updated.

ematipico · 2025-08-09T20:04:52Z

@emilyinure any updates?

coderabbitai · 2025-11-20T08:37:57Z

Walkthrough

The PR adjusts the no-accumulating-spread lint: Object.assign argument parsing now requires the first argument to be a composite literal ({}, [], new ...) to be treated as a newly allocated receiver, and the accumulator identifier is taken from the second argument. Calls where the accumulator identifier is the receiver are allowed. Test fixtures (valid/invalid) and a changeset entry reflecting the rule behaviour were updated.

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arendjr

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❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run `@coderabbitai generate docstrings` to improve docstring coverage.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: modifying the no-accumulating-spread rule to only flag Object.assign when it allocates a new object per iteration, directly addressing issue #6675.
Description check	✅ Passed	The description is relevant, explaining the motivation (Object.assign isn't inherently bad), referencing the closed issue (#6675), and describing the test plan validating the fix.
Linked Issues check	✅ Passed	The PR successfully addresses issue #6675 by refining the no-accumulating-spread rule to distinguish between in-place Object.assign (allowed) and patterns creating new objects per iteration (flagged), meeting the stated objective.
Out of Scope Changes check	✅ Passed	All changes are scoped to fixing issue #6675: rule logic updates in no_accumulating_spread.rs, test case adjustments reflecting the new behaviour, and a changeset entry documenting the patch.

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.changeset/wet-jeans-repair.md (1)

1-20: Changeset looks solid—all prior feedback addressed. ✅

The entry properly follows the contribution guide (past tense, linked issue, code blocks) and provides clear examples showing both the false positives that are now allowed and the patterns that still warn. The JavaScript examples are syntactically correct and illustrate the O(n²) allocation problem well.

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🔇 Additional comments (4)

crates/biome_js_analyze/src/lint/performance/no_accumulating_spread.rs (2)

208-227: LGTM! The logic correctly identifies allocating patterns.

The updated argument extraction logic properly distinguishes between:

In-place mutation: Object.assign(acc, bar) → first arg is identifier, returns None (not flagged)

New allocation: Object.assign({}, acc, bar) → first arg is object literal, checks second arg as reference (flagged)

The implementation correctly handles edge cases like insufficient arguments by using the ? operator throughout the iterator chain.

46-64: LGTM! Documentation examples align with the new behavior.

The updated examples correctly demonstrate:

Invalid: Object.assign([], acc, val) creates a new array each iteration

Valid: Object.assign(acc, val) mutates the accumulator in-place

crates/biome_js_analyze/tests/specs/performance/noAccumulatingSpread/valid.jsonc (1)

22-36: LGTM! Comprehensive test coverage for valid in-place mutations.

The test cases correctly cover all combinations:

Array and Object accumulators

Arrow and block return styles

Both reduce and reduceRight

All patterns use Object.assign(acc, bar) which mutates the accumulator in-place and should not be flagged.

crates/biome_js_analyze/tests/specs/performance/noAccumulatingSpread/invalid.jsonc (1)

35-57: LGTM! Thorough test coverage for allocating patterns.

The test cases correctly demonstrate the problematic patterns that allocate a new object/array on each iteration:

Object.assign({}, acc, bar) - new object literal

Object.assign([], acc, bar) - new array literal

Object.assign(new Array(1), acc, bar) - new array via constructor

These patterns all cause O(n²) time complexity and should be flagged by the rule.

.changeset/wet-jeans-repair.md

Updated tense usage at the beginning of changeset, linked back to issue. Moved examples to the inside of codeblocks Explains which cases aren't caught anymore and which are.

coderabbitai

Actionable comments posted: 1

🧹 Nitpick comments (1)

.changeset/wet-jeans-repair.md (1)
5-8: Minor punctuation improvement.

The description reads well but could be slightly clearer. The transition from "each iteration." to "Previously" might benefit from a stronger conjunction or restructuring:
-Fixed [#6675](https://github.com/biomejs/biome/issues/6675): Only flags
+Fixed [#6675](https://github.com/biomejs/biome/issues/6675): now only flags
 noAccumulatingSpread on Object.assign when a new object is being allocated on
-each iteration. Previously all cases using Object.assign with reduce parameters
+each iteration. Before, all cases using Object.assign with reduce parameters
 were warned despite not making new allocations.
This makes the before/after contrast more explicit.

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[uncategorized] ~7-~7: Possible missing comma found.
Context: ...t is being allocated on each iteration. Previously all cases using Object.assign with redu...

(AI_HYDRA_LEO_MISSING_COMMA)

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.changeset/wet-jeans-repair.md

Remove example that contains syntax error Update grammer

.changeset/wet-jeans-repair.md

Co-authored-by: Carson McManus <[email protected]>

…ew object for each iter (biomejs#7078) Co-authored-by: Carson McManus <[email protected]>

emilyinure added 2 commits July 31, 2025 13:46

fix: only flag object assign in accumulators, if it is creating a new…

3b13987

… object on every assignment biomejs#6675

fix: addictional handling for object assign inside of accumulators us…

3a2c931

…ing new keyword to allocate target objects

github-actions bot added A-Linter Area: linter L-JavaScript Language: JavaScript and super languages labels Jul 31, 2025

fix: Satisfy clippy on accumulating spread

1bff4f3

emilyinure mentioned this pull request Aug 1, 2025

Fix noaccumulatingspread object alloc biomejs/website#2815

Closed

noAccumulatingSpread changeset

0d616b5

odinr mentioned this pull request Sep 17, 2025

chore(biome): upgrade biome to v2 equinor/fusion-framework#3263

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3 tasks

ematipico self-assigned this Oct 9, 2025

Updated examples for docs on no-accumulating-spread

de4f1e3

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emilyinure force-pushed the fix-accumulating-object-creation branch from f027d8f to b69596a Compare November 20, 2025 08:48

Fix parameter name to match reduce parameter

d1df564

emilyinure force-pushed the fix-accumulating-object-creation branch from b69596a to d1df564 Compare November 20, 2025 08:49

Merge branch 'main' into fix-accumulating-object-creation

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Update noAccumulatingSpread changeset

99403fc

Updated tense usage at the beginning of changeset, linked back to issue. Moved examples to the inside of codeblocks Explains which cases aren't caught anymore and which are.

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Update changeset for noAccumulatingSpread

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Remove example that contains syntax error Update grammer

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Co-authored-by: Carson McManus <[email protected]>

dyc3 added this pull request to the merge queue Nov 21, 2025

Merged via the queue into biomejs:main with commit bb7a15c Nov 21, 2025
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github-actions bot mentioned this pull request Nov 20, 2025

ci: release #8161

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ryan-m-walker pushed a commit to ryan-m-walker/biome that referenced this pull request Nov 23, 2025

fix: only flag object assign in accumulators, if it is allocating a n…

89116cd

…ew object for each iter (biomejs#7078) Co-authored-by: Carson McManus <[email protected]>

l0ngvh pushed a commit to l0ngvh/biome that referenced this pull request Dec 21, 2025

fix: only flag object assign in accumulators, if it is allocating a n…

74b4812

…ew object for each iter (biomejs#7078) Co-authored-by: Carson McManus <[email protected]>

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fix: only flag object assign in accumulators, if it is allocating a new object for each iter #7078

fix: only flag object assign in accumulators, if it is allocating a new object for each iter #7078

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fix: only flag object assign in accumulators, if it is allocating a new object for each iter #7078

fix: only flag object assign in accumulators, if it is allocating a new object for each iter #7078

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Merging #7078 will not alter performance

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