[mahirhir]

Summary

In es-toolkit/compat, findLastIndex does not normalize a NaN fromIndex, so it diverges from lodash. findIndex received this guard in #1828; this applies the same fix to its mirror.

The bug

With fromIndex = NaN, the fromIndex < 0 check is false, so the else branch runs Math.min(NaN, arr.length - 1), which stays NaN. The slice then becomes arr.slice(0, NaN + 1) -> arr.slice(0, NaN) -> [], so the search runs over an empty array and returns -1 even when the element at index 0 matches.

Lodash coerces NaN to 0 via toInteger, so index 0 is still searched:

_.findLastIndex([1, 2, 3], x => x === 1, NaN); // => 0

Before this change:

findLastIndex([1, 2, 3], x => x === 1, NaN); // => -1  (should be 0)

Why the existing test did not catch it

The should treat falsey fromIndex values correctly test passes NaN, but with a predicate (x => x === 3) that never matches index 0, so both the old and new code return -1 there. The added test uses a predicate that matches index 0 to pin the behavior.

Fix

Same guard findIndex got in #1828:

if (Number.isNaN(fromIndex)) {
  fromIndex = 0;
}

yarn vitest run src/compat/array stays green (1011 tests).

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Project	Deployment	Actions	Updated (UTC)
es-toolkit	Ready	Preview, Comment	Jun 29, 2026 7:25pm