手写一个 Promise

  const PENDING = "pending";
  const RESOLVE = "resolve";
  const REJECT = "reject";

  function MyPromise(fn) {
    const that = this;
    that.status = PENDING; // MyPromise 内部状态
    that.value = null; // 传入 resolve 和 reject 的值
    that.resolveCallbacks = []; // 保存 then 中resolve的回调函数
    that.rejectCallbacks = []; // 保存 then 中reject的回调函数

    // resolve 函数 Promise内部调用 resolve 函数 例:new MyPromise((resolve,reject)=>{resolve(1)})
    function resolve(val) {
      if (that.status === PENDING) {
        that.status = RESOLVE;
        that.value = val;
        that.resolveCallbacks.forEach(cb => cb(that.value));
      }
    }
    // reject 函数 Promise内部调用的 reject 函数 例:new MyPromise((resolve,reject)=>{reject(1)})
    function reject(val) {
      if (that.status === PENDING) {
        that.status = REJECT;
        that.value = val;
        that.rejectCallbacks.forEach(cb => cb(that.value));
      }
    }
    // 调用传入 MyPromise 内的方法 例:new MyPromise((resolve,reject)=>{})   fn=(resolve,reject)=>{}
    try {
      fn(resolve, reject);
    } catch (error) {
      reject(error);
    }
  }
  // 在原型上添加then方法
  MyPromise.prototype.then = function(onFulfilled, onRejected) {
    const that = this;
    // 判断传入的是否为函数
    onFulfilled = typeof onFulfilled === "function" ? onFulfilled : v => v;
    onRejected =
      typeof onRejected === "function"
        ? onRejected
        : r => {
            throw r;
          };

    //如果 Promise 内部存在异步代码，调用then方法时，此时 promise 内部还是 PENDING 状态，将 then 里面的函数添加进回调数组，当异步处理完成后调用 MyPromise 内部的 resolve 或者 reject 函数
    if (that.status === PENDING) {
      that.resolveCallbacks.push(onFulfilled);
      that.rejectCallbacks.push(onRejected);
    }

    // 当 Promise 内部的状态已经为 resolve,则调用 then 里面的函数并传递值
    if (that.status === RESOLVE) {
      onFulfilled(that.value);
    }

    // 当 Promise 内部状态为 reject,则调用then里的回调函数并传递值
    if (that.status === REJECT) {
      onRejected(that.value);
    }
  };

  // 测试自己实现的Promise
  new MyPromise((resolve, reject) => {
    setTimeout(() => {
      resolve(1);
    }, 0);
  }).then(
    res => {
      console.log(res);
    },
    err => {}
  );