如何手写一个简单的 Promise？

class MyPromise {
  constructor(executor) {
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onFulfilledCallbacks = [];
    this.onRejectedCallbacks = [];

    const resolve = value => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        this.onFulfilledCallbacks.forEach(fn => fn());
      }
    };
    const reject = reason => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        this.onRejectedCallbacks.forEach(fn => fn());
      }
    };
    try { executor(resolve, reject); }
    catch (e) { reject(e); }
  }

  then(onFulfilled, onRejected) {
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v;
    onRejected = typeof onRejected === 'function' ? onRejected : e => { throw e; };
    
    return new MyPromise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            resolve(x);
          } catch (e) { reject(e); }
        });
      }
      if (this.state === 'rejected') {
        setTimeout(() => {
          try {
            const x = onRejected(this.reason);
            resolve(x);
          } catch (e) { reject(e); }
        });
      }
      if (this.state === 'pending') {
        this.onFulfilledCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onFulfilled(this.value);
              resolve(x);
            } catch (e) { reject(e); }
          });
        });
        this.onRejectedCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onRejected(this.reason);
              resolve(x);
            } catch (e) { reject(e); }
          });
        });
      }
    });
  }

  catch(onRejected) { return this.then(null, onRejected); }

  static resolve(v) { return new MyPromise(resolve => resolve(v)); }
  static reject(r) { return new MyPromise((_, reject) => reject(r)); }
}

注意：完整 Promise/A+ 实现还需处理 then 中返回 Promise 的递归解析（resolvePromise 函数）。