手写一个 Promise

var Promise = new Promise((resolve, reject) => {
  if (/*操作成功*/) {
    resolve(value);
  } else {
    reject(error);
  }
});
Promise.then(
  function (value) {
    // success
  },
  function (value) {
    // failure
  }
);

使用 class 手写一个 Promise

//创建一个 Promise 的类
class Promise {
  constructor(executer) {
    //构造函数 constructor 里面是个执行器
    this.status = "pending"; //默认的状态 pending
    this.value = undefined; //成功的值默认 undefined
    this.reason = undefined; //失败的值默认 undefined
    //状态只有在 pending 时候才能改变
    let resolveFn = value => {
      //判断只有等待时才能 resolve 成功
      if (this.status == pending) {
        this.status = "resolve";
        this.value = value;
      }
    };
    //判断只有等待时才能 reject 失败
    let rejectFn = reason => {
      if (this.status == pending) {
        this.status = "reject";
        this.reason = reason;
      }
    };
    try {
      //把 resolve 和 reject 两个函数传给执行器 executer
      executer(resolve, reject);
    } catch (e) {
      reject(e); //失败的话进 catch
    }
  }
  then(onFufilled, onReject) {
    //如果状态成功调用 onFufilled
    if ((this.status = "resolve")) {
      onFufilled(this.value);
    }
    //如果状态失败调用 onReject
    if ((this.status = "reject")) {
      onReject(this.reason);
    }
  }
}