Implement Array.filter

Easy
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Solution Guide: Implement Array.filter

Approach

Iterate through the array and invoke the callback predicate for each element. If the predicate returns a truthy value, push the original element into a result array. Elements that fail the test are skipped, producing a subset of the original array.

Complexity Analysis

Time
O(n)
Space
O(n)

Pattern

Array Polyfill

Why It Works

Filter tests each element against a predicate and includes only those that pass. Passing Boolean as the callback is a common idiom to remove all falsy values (0, "", null, undefined, false).

Updated Feb 2026