### 背景
Promise是异步编程的一种解决方案，它可以解决异步回调地狱的问题，防止层层嵌套对程序代码带来的难维护性。既然带来了方便，我们就有必要学习它的原理以及底层实现，所以笔者就按照PromiseA+规范写了一个简单的Promise，并实现了Promise.all()，Promise.race()等API
### 实现过程
1.定义Promise，并传入一个需要执行的task函数，以及Promise中非常重要的三种状态
```
//定义Promise的三种状态
const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';
function Promise(executor){}
```
2.设置默认状态，并定义成功和失败的回调函数数组（为了解决链式调用的问题）
```
//设置默认状态
self.status = PENDING;
//存放成功的回调函数的数组
self.onResolvedCallbacks =[];
//定义存放失败回调函数的数组
self.onRejectedCallbacks =[];
```
3.定义成功和失败的回调函数实现
```
function resolve(value){
if(value!=null &&value.then&&typeof value.then == 'function'){
return value.then(resolve,reject);
}
// This can be implemented with either a“macro-task”mechanism such as setTimeout or setImmediate,or with a“micro-task”mechanism such as MutationObserver or process.nextTick. Since the promise implementation is considered platform code
setTimeout(function(){
if(self.status == PENDING){
self.status = FULFILLED;
self.value = value;
self.onResolvedCallbacks.forEach(cb=>cb(self.value));
}
})
}
// When rejected,a promise:
// must not transition to any other state.
// must have a reason,which must not change.
function reject(reason){
setTimeout(function(){
if(self.status == PENDING){
self.status = REJECTED;
self.value = reason;
self.onRejectedCallbacks.forEach(cb=>cb(self.value));
}
});
}
```
4.实现then方法，这个很重要，就是异步任务执行成功调用then方法，依次走下去，避免了回调黑洞,其中resolvePromise严格按照[PromiseA+规范](https://promisesaplus.com/)第2.3条去实现
```
Promise.prototype.then = function(onFulfilled,onRejected){
onFulfilled = typeof onFulfilled == 'function'?onFulfilled:function(value){return value};
onRejected = typeof onRejected == 'function'?onRejected:reason=>{throw reason};
let self = this;
let promise2;
if(self.status == FULFILLED){
return promise2 = new Promise(function(resolve,reject){
setTimeout(function(){
try{
let x =onFulfilled(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
})
});
}
if(self.status == REJECTED){
return promise2 = new Promise(function(resolve,reject){
setTimeout(function(){
try{
let x =onRejected(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
})
});
}
if(self.status == PENDING){
return promise2 = new Promise(function(resolve,reject){
self.onResolvedCallbacks.push(function(){
try{
let x =onFulfilled(self.value);
//如果获取到了返回值x,会走解析promise的过程
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
});
self.onRejectedCallbacks.push(function(){
try{
let x =onRejected(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
});
});
}
}
function resolvePromise(promise2,x,resolve,reject){
if(promise2 === x){
return reject(new TypeError('构成循环引用'));
}
//promise2是否已经resolve或reject了
let called = false;
if(x instanceof Promise){
if(x.status == PENDING){
x.then(function(y){
resolvePromise(promise2,y,resolve,reject);
},reject);
}else{
x.then(resolve,reject);
}
//x是一个thenable对象或函数，只要有then方法的对象，
}else if(x!= null &&((typeof x=='object')||(typeof x == 'function'))){
try{
let then = x.then;
if(typeof then == 'function'){
then.call(x,function(y){
if(called)return;
called = true;
resolvePromise(promise2,y,resolve,reject)
},function(err){
if(called)return;
called = true;
reject(err);
});
}else{
//x不是一个thenable对象
resolve(x);
}
}catch(e){
if(called)return;
called = true;
reject(e);
}
}else{
resolve(x);
}
}
```
5.Promise.all方法用于将多个 Promise 实例，包装成一个新的 Promise 实例。只有所有实例的状态都变成fulfilled，最后的状态才会变成fulfilled，此时返回值组成一个数组，传递给最终的回调函数。
```
function gen(times,cb){
let result =[],count=0;
return function(i,data){
result[i]= data;
if(++count==times){
cb(result);
}
}
}
Promise.all = function(promises){
return new Promise(function(resolve,reject){
let done = gen(promises.length,resolve);
for(let i=0;i
promises[i].then(function(data){
done(i,data);
},reject);
}
});
}
```
6.Promise.race方法同样是将多个 Promise 实例，包装成一个新的 Promise 实例。但是只要多个实例之中有一个实例率先改变状态，最终的状态就跟着改变。那个率先改变的 Promise 实例的返回值，就传递给最终的回调函数。
```
Promise.race = function(promises){
return new Promise(function(resolve,reject){
for(let i=0;i
promises[i].then(resolve,reject);
}
});
}
```
### 参考链接
1.[PromiseA+规范](https://promisesaplus.com/)
2.[PromiseA+](https://segmentfault.com/a/1190000002452115)
