findIndex(): zero is a valid result

Useful to know

findIndex() searches for the first element that fulfills a condition, and returns its index. For example, [8, 2, 5].findIndex(x => x < 4) gives 1. If no match, returns -1. [1]

Do not use if (index) as a check for an item found. Index 0 is valid but interpreted as false; -1 is interpreted as true. The index !== -1 condition expresses the correct verification. This is especially important if you are going to delete item by position after the search.