promise构造器的实现

const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';

class MyPromise {
  #state = PENDING;
  #result = undefined;
  #handlers = [];

  constructor(executor) {
    const resolve = (data) => {
      this.#changeState(FULFILLED, data);
    };
    const reject = (reason) => {
      this.#changeState(REJECTED, reason);
    };
    try {
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }

  #changeState(state, result) {
    if (this.#state !== PENDING) return;
    this.#state = state;
    this.#result = result;
    this.#run();
  }

  #isPromiseLike(value) {
    return (value !== null && (typeof value === 'object' || typeof value === 'function') && (typeof value.then === 'function'));
  }

  // 把传递的函数放入微队列中
  #runMicroTask(func) {
    // 判断node环境
    if (process && process.nextTick) {
      process.nextTick(func);
    } else if (MutationObserver) {
      const p = document.createElement('p');
      const observer = new MutationObserver(func);
      observer.observe(p, {
        childList: true
      });
      p.innerHTML = 1;
    } else {
      setTimeout(func, 0);
    }
  }

  #runOne(callback, resolve, reject) {
    this.#runMicroTask(() => {
      if (typeof callback === 'function') {
        const settled = this.#state === FULFILLED ? resolve : reject;
        settled(this.#result);
        return;
      }
      try {
        const data = callback(this.#result);
        // 判断返回值是否promise
        if (this.#isPromiseLike(data)) {
          data.then(resolve, reject);
        } else {
          resolve(data);
        }
        resolve(data);
      } catch (err) {
        reject(err);
      }
    });
  }

  #run() {
    if (this.#state === PENDING) return;
    while (this.#handlers.length) {
      const {onFulfilled, onRejected, resolve, reject} = this.#handlers.shift();
      if (this.#state === FULFILLED) {
        this.#runOne(onFulfilled, resolve, reject);
      } else {
        this.#runOne(onRejected, resolve, reject);
      }
    }
  }

  then(onFulfilled, onRejected) {
    return new MyPromise((resolve, reject) => {
      this.#handlers.push({
        onFulfilled,
        onRejected,
        resolve,
        reject
      });
      this.#run();
    });
  }
}

const p = new Promise((resolve, reject) => {
  throw 123;
});

console.log(p);