Implement Custom Promise Polyfill (Promises/A+ Microtask Chaining)

Problem Statement

Implement a custom `MyPromise` class from scratch supporting constructor executor, `.then()`, `.catch()`, chaining, and asynchronous microtask execution.

Target Complexity

Time:

O(1) resolve/reject dispatch

Space:

O(N) for callback subscriber queues

Interview Talking Points:

• Explain the 3 Promise states: 'pending', 'fulfilled', and 'rejected' (transitions are one-way and immutable).
• Explain why `.then()` returns a NEW Promise instance rather than returning `this` (enabling independent branches and value transformation).

Solution Editor (JavaScript)

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Test Results

Resolves asynchronously and chains values through .then()

Test #1

Propagates errors to .catch() handler

Test #2