promise 拙见

const promise = new Promise(function(resolve, reject) {
      // ... some code

      if (/* 异步操作成功 */){
        resolve(value);
      } else {
        reject(error);
      }
  });

    promise.then(function(value) {
      // success
    }, function(error) {
      // failure
    });

// 第一步：Promise构造函数接受一个函数作为参数，该函数的两个参数分别是resolve和reject。它们是两个函数，由 JavaScript 引擎提供，不用自己部署。
    function Promise(task) {

        let that = this; // 缓存this
        that.status = 'pending'; // 进行中的状态
        that.value = undefined; //初始值

        that.onResolvedCallbacks = []; // 存放成功后要执行的回调函数的序列
        that.RejectedCallbacks = []; // 存放失败后要执行的回调函数的序列
        // 该方法是将Promise由pending变成fulfilled
        function resolve (value) {
            if (that.status == 'pending') {
                that.status = 'fulfilled';
                that.value = value;
                that.onResolvedCallbacks.forEach(cb => cd(that.value))
            }

        }
        // 该方法是将Promise由pending变成rejected
        function reject (reason) {
          if (that.status == 'pending') {
                that.status = 'rejected';
                that.value = reason;
                that.onRjectedCallbacks.forEach(cb => cd(that.value))
            }
        }

        try {
        // 每一个Promise在new一个实例的时候 接受的函数都是立即执行的
            task(resolve, reject)
        } catch (e) {
            reject(e)
        }
    }

// 第二部 写then方法，接收两个函数onFulfilled onRejected，状态是成功态的时候调用onFulfilled 传入成功后的值，失败态的时候执行onRejected，传入失败的原因，pending 状态时将成功和失败后的这两个方法缓存到对应的数组中，当成功或失败后 依次再执行调用
    Promise.prototype.then = function(onFulfilled, onRejected) {
        let that = this;
        if (that.status == 'fulfilled') {
            onFulfilled(that.value);
        }
        if (that.status == 'rejected') {
            onRejected(that.value);
        }
        if (that.status == 'pending') {
            that.onResolvedCallbacks.push(onFulfilled);
            that.onRjectedCallbacks.push(onRejected);
        }
    }

    let promise = new Promise(function (resolve, reject) {
        resolve(100);// reject(100)
    });

    promise.then(function (data) {
        return data+100;

    },function (err) {
       return 'ssss';

    }).then(function (data) {
        console.log(data);// 200  // undefined // sss
    })

    //catch原理就是只传失败的回调
    Promise.prototype.catch = function(onRejected){
        this.then(null,onRejected);
    }

  Promise.all = function(promises){
     return new Promise(function(resolve,reject){
       let done = gen(promises.length,resolve);
       for(let i=0;i<promises.length;i++){
         promises[i].then(function(data){
           done(i,data);
         },reject);
       }
     });
    }

    //返回一个立刻成功的promise
    //别人提供 给你一个方法，需要你传入一个promise,但你只有一个普通的值，你就可以通过这个方法把这个普通的值(string number object)转成一个promise对象
    Promise.resolve = function(value){
      return new Promise(function(resolve){
        resolve(value);
      });
    }

 //返回一个立刻失败的promise
    Promise.reject = function(reason){
      return new Promise(function(resolve,reject){
        reject(reason);
      });
    }

    Promise.race = function(promises){
      return new Promise(function(resolve,reject){
        for(let i=0;i<promises.length;i++){
          promises[i].then(resolve,reject);
        }
      });
    }