Implement a minimal Promise/A+ (MyPromise) with then/catch

Question

Accepted Answer

**Approach:** States pending/fulfilled/rejected; queue handlers; `resolvePromise` handles thenables recursively; `then` returns new promise.

```javascript
class MyPromise {
  constructor(executor) {
    this.state = 'pending'; this.value = undefined; this.handlers = [];
    const resolve = (v) => this._resolve(v);
    const reject = (r) => this._reject(r);
    try { executor(resolve, reject); } catch(e) { reject(e); }
  }
  _resolve(v) {
    if (this.state !== 'pending') return;
    if (v && (typeof v === 'object' || typeof v === 'function') && typeof v.then === 'function') {
      try { v.then(this._resolve.bind(this), this._reject.bind(this)); } catch(e){ this._reject(e); }
      return;
    }
    this.state = 'fulfilled'; this.value = v;
    this.handlers.forEach(h => this._handle(h));
  }
  _reject(r) {
    if (this.state !== 'pending') return;
    this.state = 'rejected'; this.value = r;
    this.handlers.forEach(h => this._handle(h));
  }
  _handle(handler) {
    if (this.state

Implement a minimal Promise/A+ (MyPromise) with then/catch

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Understand the problem

Attempt it yourself

Study the solution

Solution ready — 2 min read

Join the discussion

Discussion (0)

KEEP THESTREAK ALIVE.

KEEP THE
STREAK ALIVE.