Implement a custom MyPromise class that reproduces the core behaviour of the native JavaScript Promise.
Requirements
- Constructor accepts an executor
(resolve, reject) => void.
- Support instance methods:
then(onFulfilled?, onRejected?)catch(onRejected?)finally(onFinally?)
- Handlers must run asynchronously (micro-task style).
- Each
then returns a new promise to enable chaining.
- Handle edge-cases:
- Errors thrown inside executor or callbacks.
- Returning a promise/thenable from within
then.
finally executes on both resolve and reject.
Example
const p = new MyPromise((resolve) => {
setTimeout(() => resolve(42), 100);
});
p.then(v => v * 2)
.then(console.log); // 84 (after ~100 ms)
new MyPromise((_, reject) => reject('error'))
.catch(console.log); // "error"