Implement Array.indexOf

Easy
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Solution Guide: Implement Array.indexOf

Approach

Linear scan from fromIndex using strict equality to find the first matching element. Handle negative fromIndex by computing the offset from the end. Return the index on the first match, or -1 if the value is not found in the remaining elements.

Complexity Analysis

Time
O(n)
Space
O(1)

Pattern

Array Polyfill

Why It Works

IndexOf returns the numeric position of the first occurrence, enabling operations like removing duplicates, finding positions for splice, or checking existence before includes was introduced.

Updated Feb 2026