How are the functions indexOf and findIndex of an array different from each other?

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The main difference between the functions indexOf and findIndex of an array is that indexOf returns the first index at which a given element can be found in the array whereas findIndex returns the index of the first element in the array that satisfies a given testing function.

So, indexOf searches for a specific element in the array and returns its index (or -1 if not found), whereas findIndex searches the array for the first element that meets a specific criteria (determined by the testing function) and returns its index (or -1 if no element satisfies the criteria).