fix/compat array fill iteratee guard

[kojesung]

Summary

• Previously, es-toolkit/compat's fill appeared to fill each element of the inner arrays with the index the same as lodash when used as a .map callback on a 2D array like [[1, 2], [3, 4]]. However, this wasn't because an isIterateeCall guard existed; it succeeded only by coincidence, because the outer array passed in by .map turned into NaN after going through Math.floor(), and was then coincidentally reset to 0 by the if (!start) start = 0 handling in fill.ts. A test case was added for an exceptional situation where this coincidence does not hold (when the outer array has a single element).
• When fill is used directly as a .map callback (array.map(fill)), lodash resets start/end to their default values via the isIterateeCall guard, but es-toolkit/compat's fill lacked this guard, causing it to behave differently from lodash in certain cases. Specifically, when the outer array has a single element ([[5]].map(fill)), Number([[5]]) coincidentally converts to 5, and start=5 ends up exceeding the array's length (1), so nothing gets filled — a bug. To prevent this case, the fix reuses the same isIterateeCall helper already used in every/slice/...

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Project	Deployment	Actions	Updated (UTC)
es-toolkit	Ready	Preview, Comment	Jul 29, 2026 12:44am