sindresorhus/eslint-plugin-unicorn

`no-useless-spread` it is not safe to remove spread in `[...foo.concat(bar)]`

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#2,480 opened on Oct 16, 2024

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Description

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/2b469bee475a8f3f2767f4669864acdd89654017/test/no-useless-spread.mjs#L259

[!NOTE] The concat() method preserves empty slots if any of the source arrays is sparse.

via MDN

const foo = new Array(5);
const bar = new Array(5);

const cat = [...foo.concat(bar)];
// now we got a new array with undefined x 10
// but foo.concat(bar) is empty x 10.
// It's not what we want.

Contributor guide

Research direction
Investigate the `no useless spread` rule logic to understand how it detects removable spreads. Trace the test case at `test/no useless spread.mjs` line 259 to see the failing scenario. Check MDN documentation for `Array.prototype.concat` behavior with sparse arrays to confirm the spread removal is unsafe. Consider adding a check for sparse arrays in the rule.
Tech stack
javascript
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
JavaScriptESLint
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60