fix/compat array find array like length

[kojesung]

Summary

• The existing find used the if (!Array.isArray(source)) statement to treat anything that wasn't an array as an object and process it accordingly. However, this branch handling, when an object has a readable property called length, uses the property name length as a key to perform the search. But this differs from lodash's intended behavior. When lodash encounters a length property — such as the length that exists on real arrays — it does not use it as a key and ignores it instead.

const arrayLike = { 0: 'a', 1: 'b', length: 2 };
console.log(findCompat(arrayLike, v => v === 2));
// 2

console.log(lodash.find(arrayLike, v => v === 2));
// undefined

• To follow this behavior of lodash, instead of Array.isArray, I applied the implementation approach of .map, which uses the isArrayLike function to extract keys excluding the length property when it exists, and treats only the extracted keys as the iteration target. Through this, I implemented it so that, just like lodash's implementation, when a length property exists it is not used as a key and is ignored.

• In this process, I also removed the structure where the array branch delegated to native code via .slice(fromIndex).find(doesMatch); as a side effect, this also fixed the index/collection arguments passed to the predicate when using fromIndex so that they are now correctly passed based on the original array (rather than the sliced array).

• I added a test case that can verify this work.

• As the implementation approach changed, the shape of the error message that the 'should throw error when boolean predicate is used' test was verifying ended up changing. Because the entity that throws the error changed, I modified the assertion message so the test would pass. For reference, actual lodash does not throw an error for a boolean predicate and instead returns undefined; this behavior itself (throwing an error) was judged to be the domain of the iteratee function, so it was not included in the scope of this fix.

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Project	Deployment	Actions	Updated (UTC)
es-toolkit	Ready	Preview, Comment	Jul 29, 2026 12:55am