#430 Custom Promise

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"