首先，先来看Promise用法，Promise主要用于异步，在之前的JS，除了上传图片，以及Ajax，几乎没有要用到异步的地方。但是Node.JS的出现，改变了这一现象，因为在Node出现的时候，后端语言，Java，Python，PHP已经占据了后端的主要份额，要想脱引而出，就要有一些其他语言没有的特点，Node的特点，就是大量回调来处理并发， 所以会用到大量的回调函数，Promise就登场了，当然，ES7出现的asyns和await更是将回调完美的展现出来，这点在Koa2中，大量运用。Promise究竟为何物，我们先来揭开Promise的用法吧：
不用Promise的情况：
const ajax = function(callback){
console.log("start");
setTimeout(()=>{
callback&&callback.call();
},1000)
}
ajax(function(){
console.log("Hello World");
})
我们定义一个ajax方法，里面传入一个方法，用setTimeout模拟发送请求的操作，然后在一秒钟，输出这个方法，结果也输出正确。
Promise基本使用：
const ajax = function(){
console.log("start");
return new Promise(function(resolve,reject){
setTimeout(()=>{
resolve("hello");
},1000)
})
}
ajax().then((value)=>{
console.log(value,"world");
})
和上文 一样，只不过这次是返回一个Promise对象，Promise对象可以链式调用then方法，进行下一步操作。
Promise链式调用：
const ajax = function(){
console.log("start");
return new Promise(function(resolve,reject){
setTimeout(function(){
resolve()
},1000);
})
}
ajax().then(function(){
return new Promise(function(resolve,reject){
setTimeout(function(){
resolve()
},2000);
})
}).then(()=>{
console.log("Hello World");
})
注意，then方法，会默认返回一个Promise对象，比如return 1,这个1，就对应在下次then(value=>{})的value值，但是这样功能太少了，比如说抛出错误，不好维护，所以还是应该写return new Promise();
Promise链式调用的复杂情形：
new Promise(resolve=>{
console.log("start1");
setTimeout(()=>{
resolve(100);
},1000);
})
.then(value=>{
return new Promise(resolve=>{
console.log("start2");
console.log(value);
setTimeout(()=>{
resolve(200);
},1000);
})
})
.then(value=>{
return new Promise(resolve=>{
console.log("start3");
console.log(value);
setTimeout(()=>{
resolve(300);
},1000);
})
})
第一个方法，返回的结果参数，会被第二个方法执行，第二个方法返回的参数会被第三个执行。
Promise报错的处理：
const ajax = function(num){
console.log("start");
return new Promise(function(resolve,reject){
if(num>5){
resolve();
}else{
throw new Errow("出错了");
}
})
}
ajax(3).then(()=>{
console.log("Hello World");
}).catch((err)=>{
console.log(err);
})
建议即使只需要then，还是应该写catch，捕获异常。
Promise高级篇：
所有图片加载完再添加到页面：
function loadImg(src) {
return new Promise((resolve, reject) => {
let img = document.createElement('img');
img.src = src;
img.onload = function() {
resolve(img);
}
img.onerror = function(err) {
reject(err);
}
})
}
function showImgs(imgs) {
imgs.forEach(function(img) {
document.body.appendChild(img);
})
}
Promise.all([
loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1559449634314&di=56a92121182ba40c4a640c053dc0c64b&imgtype=0&src=http%3A%2F%2Fb.hiphotos.baidu.com%2Fimage%2Fpic%2Fitem%2F9825bc315c6034a8ef5250cec5134954082376c9.jpg'),
loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1559449650753&di=5642d7deef9e1e63b6f3868c75b625f0&imgtype=0&src=http%3A%2F%2Fg.hiphotos.baidu.com%2Fimage%2Fpic%2Fitem%2Fc2cec3fdfc03924590b2a9b58d94a4c27d1e2500.jpg'),
loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1559449650745&di=6d65c12f144a14bb2593901da1e995a1&imgtype=0&src=http%3A%2F%2Fh.hiphotos.baidu.com%2Fimage%2Fpic%2Fitem%2F0b46f21fbe096b63491b16ea06338744ebf8ac0e.jpg')
]).then(showImgs)
有一个图片加载完就添加到页面：
function loadImg(src){
return new Promise((resolve,reject)=>{
let img=document.createElement('img');
img.src=src;
img.onload=function(){
resolve(img);
}
img.onerror=function(err){
reject(err);
}
})
}
function showImgs(img){
let p=document.createElement('p');
p.appendChild(img);
document.body.appendChild(p)
}
Promise.race([
loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1559449634314&di=56a92121182ba40c4a640c053dc0c64b&imgtype=0&src=http%3A%2F%2Fb.hiphotos.baidu.com%2Fimage%2Fpic%2Fitem%2F9825bc315c6034a8ef5250cec5134954082376c9.jpg'),
loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1559449650753&di=5642d7deef9e1e63b6f3868c75b625f0&imgtype=0&src=http%3A%2F%2Fg.hiphotos.baidu.com%2Fimage%2Fpic%2Fitem%2Fc2cec3fdfc03924590b2a9b58d94a4c27d1e2500.jpg'),
loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1559449650745&di=6d65c12f144a14bb2593901da1e995a1&imgtype=0&src=http%3A%2F%2Fh.hiphotos.baidu.com%2Fimage%2Fpic%2Fitem%210b46f21fbe096b63491b16ea06338744ebf8ac0e.jpg')
]).then(showImgs)
实现队列：
function queue(things) {
let promise = Promise.resolve();
things.forEach(element => {
promise = promise.then(() => {
return new Promise(resolve => {
setTimeout(() => {
console.log(element)
resolve('ok');
}, 1000);
});
})
});
}
queue(['a', 'b', 'c']);
可依靠这个实现JQuery的animate队列。
ES7写法：
async function queue(arr) {
let res = null
for (let promise of arr) {
res = await promise(res)
}
return await res
}
queue(["a", "b", "c"])
.then(data => {
console.log(data)// abc
})
Promise.reduce也可以用来顺序执行函数，但是可使用的场景非常有限，一般用来读取文件信息。
手写Promise实现：
const PENDING = "pending";
const RESOLVED = "resolved";
const REJECTED = "rejected";
function MyPromise(fn){
let that = this;
that.state = PENDING;
that.value = null;
that.resolvedCallbacks = [];
that.rejectedCallbacks = [];
function resolve(value){
if(that.state === PENDING){
that.state = RESOLVED;
that.value = value;
that.resolvedCallbacks.map(callback=>callback(that.value))
}
}
function reject(value){
if(that.state === PENDING){
that.state = REJECTED;
that.value = value;
that.rejectedCallbacks.map(callback=>callback(that.value))
}
}
try{
fn(resolve,reject)
}catch(e){
reject(e)
}
}
MyPromise.prototype.then = function(onFulfilled,onRejected){
let that = this;
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v;
onRejected = typeof onRejected === 'function' ? onRejected : r=>{throw r};
if(that.state === PENDING){
that.resolvedCallbacks.push(onFulfilled)
that.rejectedCallbacks.push(onRejected)
}
if(that.state === RESOLVED){
onFulfilled(that.value);
}
if(that.state === REJECTED){
onRejected(that.value);
}
}
const ajax = function(){
console.log("start");
return new MyPromise(function(resolve,reject){
setTimeout(()=>{
resolve("hello");
},1000)
})
}
ajax().then((value)=>{
console.log(value,"world");
})
