SOURCE

//promise用法
/*
var p = new Promise(function (resolve, reject) {
    setTimeout(function () {
        // A动画
        console.log('A');
        resolve();
    }, 300);
});

p.then(function () {
    setTimeout(function () {
        // B动画
        console.log('B');
    }, 300);
});
*/

//实现promise
class PromiseNew{
  constructor(fn){
    this.state = 'PENDING';
    this.doneList = [];
    this.failList = [];
    fn(this.resolve.bind(this),this.reject.bind(this));
  }
  
  //注册成功处理函数
  done(handle) {
    if(typeof handle === 'function'){
      this.doneList.push(handle);
    }else{
      throw new Error('缺少回调函数');
    }
    return this;
  }
  
  //注册失败处理函数
  fail(handle) {
    if(typeof handle === 'function') {
      this.failList.push(handle);
    }else {
      throw new Error('缺少回调函数');
    }
    return this;
  }
  
  //同时注册成功和失败处理函数
  then(success,fail) {
    this.done(success || function () { }).fail(fail || function(){ });
    return this;
  }
  
  //更新state为RESOLVED，并且执行成功处理队列
  resolve() {
    this.state = 'RESOLVED';
    let args = Array.prototype.slice.call(arguments);
    setTimeout(function () {
      this.doneList.forEach((item,key,arr)=>{
        item.apply(null,args);
        arr.shift();
      });
    }.bind(this),200)
  }
  
  //更新state为REJECTED，并且执行失败处理队列
  reject() {
    this.state = 'REJECTED';
    let args = Array.prototype.slice.call(arguments);
    setTimeout(function(){
      this.failList.forEach((item,key,arr)=>{
        item.apply(null,args);
        arr.shift();
      });
    }.bind(this),200)
  }
}

new PromiseNew((resolve, reject) => {
  resolve('hello world');
  // reject('you are err');
}).done((res) => {
  console.log(res);
}).fail((res) => {
  console.log('error');
})