What is the time and space complexity of Create Your Own Promise?

Time complexity: O(n) where n is the handler chain length. Space complexity: O(n).

Create Your Own Promise

Hard

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Solution Guide: Create Your Own Promise

Approach

Implement a simplified Promise class with pending/fulfilled/rejected states and a handlers queue. The constructor runs the executor synchronously, calling resolve or reject to transition state. The then method returns a new MyPromise, queuing handlers if still pending or scheduling them asynchronously if already settled.

Complexity Analysis

Time: O(n) where n is the handler chain length
Space: O(n)

Pattern

Promise Implementation

Why It Works

State machine semantics ensure a promise transitions only once, and deferred handler execution via the queue guarantees then callbacks registered before settlement are invoked when the value becomes available.

Updated Feb 2026