Build a mini Promise from scratch (MyPromise)

Build MyPromise from scratch

The deep-end interview question: implement a minimal, spec-inspired Promise class yourself. MyPromise should support:

• new MyPromise(executor) where executor(resolve, reject) runs synchronously.
• .then(onFulfilled, onRejected) returning a new MyPromise, so chains work.
• .catch(onRejected) as sugar for .then(undefined, onRejected).
• Once settled, the state is locked in (calling resolve/reject again does nothing).
• A .then registered after the promise has already settled still fires (with its result delivered asynchronously, a microtask, not a synchronous call).
• Because your .then(onFulfilled, onRejected) has the same shape the language expects, real await will actually work on a MyPromise, you don't need to do anything special for that part.

You do not need to handle a callback returning another thenable/promise (no recursive resolution), keep it to plain values.

const p = new MyPromise((resolve) => resolve(42));
await p; // 42
p.then((v) => v * 2).then((v) => console.log(v)); // 84