@studnicky/v8/array-from-map-callback

Disallows Array.from(iterable, mapFn). The rule resolves ArrayConstructor.from from the standard library, covering computed or aliased calls while excluding an unrelated static method named from. It applies at every call site, not only in loops: the measured cost is proportional to the source size even for one top-level call.

For a 5,000,000-element array in Node v24, Array.from(arr, mapFn) takes 79.28 ms and new Array(n) with indexed assignment takes 6.27 ms, a 12.65× difference. Use an index-assignment loop where the mapped result is needed, or use the one-argument form when conversion is sufficient.

Fixable: No · Options: No · Suggested severity: error

✗ Incorrect

const doubled = Array.from(values, (value) => value * 2);

const labels = Array.from(records, (record) => record.label);

✓ Correct

const doubled = new Array<number>(values.length);
for (let index = 0; index < values.length; index += 1) {
  doubled[index] = (values[index] ?? 0) * 2;
}

const copied = Array.from(values);