[Grit03]

Summary

findLastIndex() in es-toolkit/compat does not run fromIndex through toInteger() the way Lodash does, so a negative fraction or a non-numeric value makes the search start from a different index.

const array = [1, 2, 3, 1, 2, 3];

findLastIndex(array, x => x === 3, -1.5);
// Lodash: 5, es-toolkit/compat: 2

findLastIndex(array, x => x === 1, '-2');
// Lodash: 3, es-toolkit/compat: -1

findLastIndex(array, x => x === 1, 'abc');
// Lodash: 0, es-toolkit/compat: -1

The difference comes from where the truncation happens.

• lodash.js converts fromIndex with toInteger() before adding it to arr.length.
• es-toolkit/compat adds the raw value instead.

With -1.5 on a six-element array, lodash.js starts at 6 + (-1) = 5, while es-toolkit/compat gets 6 + (-1.5) = 4.5 and slice(0, 4.5 + 1) stops at index 4, one short.

Non-numeric values break for a different reason. + concatenates instead of adding, so 6 + '-2' is '6-2', which is NaN and leaves nothing to search.

Changes

• Convert fromIndex with toInteger before using it to compute the search range.
• Add tests for negative fractional and non-numeric fromIndex.

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Project	Deployment	Actions	Updated (UTC)
es-toolkit	Ready	Preview	Aug 22, 2026 1:41pm