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

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

Implement Array.join

Easy

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Solution Guide: Implement Array.join

Approach

Iterate through the array, concatenating each element to a result string with the separator inserted between elements. Handle the default comma separator when none is provided, and treat null/undefined values as empty strings to match native behavior.

Complexity Analysis

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

Pattern

Array Polyfill

Why It Works

String concatenation with a conditional separator between elements reproduces the native join behavior, including the edge case of skipping null and undefined values.

Updated Feb 2026