What is the time and space complexity of Implement Array.reduce?

Time complexity: O(n). Space complexity: O(1).

Implement Array.reduce

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Solution Guide: Implement Array.reduce

Approach

Initialize the accumulator to the provided init value (or the first element if omitted, starting iteration from index 1). On each iteration, pass the accumulator and current element to the callback, updating the accumulator with the return value. Return the final accumulator after the loop.

Complexity Analysis

Time: O(n)
Space: O(1)

Pattern

Array Polyfill

Why It Works

Reduce collapses an array into a single value by threading an accumulator through each iteration. The flexibility of the callback allows it to implement sum, max, flatten, groupBy, and nearly any array transformation.

Updated Mar 2026