Implement Array.findIndex

Easy
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Solution Guide: Implement Array.findIndex

Approach

Loop through the array and test each element against the callback predicate. Return the index of the first element that passes the test, short-circuiting the iteration. Return -1 if no element satisfies the condition.

Complexity Analysis

Time
O(n)
Space
O(1)

Pattern

Array Polyfill

Why It Works

FindIndex returns a numeric position rather than the element itself, which is useful when you need to know where a match occurs for subsequent splice, slice, or index-based operations.

Updated Feb 2026