Promise表示一个异步操作的最终结果。与Promise最主要的交互方法是通过将函数传入它的then方法从而获取得Promise最终的值或Promise最终最拒绝（reject）的原因。
Promise解决的问题是以往在js函数中使用回调函数的问题。
没有Promise之前：
function personInfo(callback){
//dosometing
callback&&callback();
}
原生Promise用法
let p=new Promise((resolve,reject)=>{resolve('123');
});
p.then(data=>{console.log(data)});
按照Promise/A+规范来实现一个Promise类。
Promise有三个状态：pending, fulfilled 或 rejected。
初始化状态为pending
成功状态为fulfilled
失败状态rejected
self.value存放的是成功的值
self.reason存放的是失败的值
self.onResolvedCallbacks存的是所有成功回调的值
self.onRejectedCallbacks存的是所有失败回调的值
function Promise(executor){
let self=this;
self.status='pending';
self.value=undefined;
self.reason=undefined;
self.onResolvedCallbacks=[];
self.onRejectedCallbacks=[];
function resolve(value){
//new Promise(new Promise((resolve,reject)=>resolve()));
if(value!=null &&value.then&&typeof value.then == 'function'){
return value.then(resolve,reject);
}
setTimeout(function(){
if(self.status=='pending'){
self.status='fulfilled';
self.value=value;
self.onResolvedCallbacks.forEach(item=>item(self.value));
}
});
}
function reject(reason){
setTimeout(function(){
if(self.status=='pending'){
self.status='rejected';
self.reason=reason;
self.onRejectedCallbacks.forEach(item=>item(self.reason));
}
});
}
try{
executor(resolve,reject);
}catch(e){
reject(e);
}
}
构造函数原型上的then方法，通过状态获取值。返回一个新的Promise实例。
Promise.prototype.then=function(onFulfilled,onRejected){
onFulfilled=typeof onFulfilled=='function'?onFulfilled:value=>value;
onRejected=typeof onRejected=='function'?onRejected:reason=>{throw reason};
let self=this
let promise2;
if(self.status=='resolve'){
return promise2=new Promise((resolve,reject)=>{
try{
let x=onFulfilled(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e)
}
});
}
if(self.status=='reject'){
return promise2=new Promise((resolve,reject)=>{
try{
let x = onRejected(self.reason);
resolvePromise(promise2, x, resolve, reject);
}catch(e){
reject(e)
}
});
}
if(self.status=='pending'){
return promise2 = new Promise(function (resolve, reject) {
self.onResolvedCallbacks.push(()=>{
try {
let x = onFulfilled(self.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
});
self.onRejectedCallbacks.push(()=>{
try {
let x = onRejected(self.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
});
})
}
}
错误捕获,走失败的回调函数
Promise.prototype.catch = function(onRejected){this.then(null,onRejected);
}
Promise.all所有的Promise执行完，再执行下一步。接受的参数是个数组，返回有一个新的Promise实例。数组中的每一项必须为成功态，then得到的值也会依次按顺序排列在数组中
Promise.all=function(promises){ return new Promise((resolve,reject)=>{ let result=[]; let count=0; let len=promises.length; for(let i=0;i{result[i]=data;
if(++count==len){
resolve(result);
}
},reject);
}
});
}
Promise.race一个Promise执行完，就执行下一步。接受的参数是个数组，返回有一个新的Promise实例。