ES6 原理实现
Promise A+ 实现
js
function resolvePromise(promise2,x,resolve,reject){
//判断x是不是promise
//规范中规定：我们允许别人乱写，这个代码可以实现我们的promise和别人的promise 进行交互
if(promise2 === x){//不能自己等待自己完成
return reject(new TypeError('循环引用'));
};
// x是除了null以外的对象或者函数
if(x !=null && (typeof x === 'object' || typeof x === 'function')){
let called;//防止成功后调用失败
try{//防止取then是出现异常 object.defineProperty
let then = x.then;//取x的then方法 {then:{}}
if(typeof then === 'function'){//如果then是函数就认为他是promise
//call第一个参数是this，后面的是成功的回调和失败的回调
then.call(x,y => {//如果Y是promise就继续递归promise
if(called) return;
called = true;
resolvePromise(promise2,y,resolve,reject)
},r => { //只要失败了就失败了
if(called) return;
called = true;
reject(r);
});
}else{//then是一个普通对象，就直接成功即可
resolve(x);
}
}catch (e){
if(called) return;
called = true;
reject(e)
}
}else{//x = 123 x就是一个普通值 作为下个then成功的参数
resolve(x)
}
}
class Promise {
constructor (executor){
//默认状态是等待状态
this.status = 'panding';
this.value = undefined;
this.reason = undefined;
//存放成功的回调
this.onResolvedCallbacks = [];
//存放失败的回调
this.onRejectedCallbacks = [];
let resolve = (data) => {//this指的是实例
if(this.status === 'pending'){
this.value = data;
this.status = "resolved";
this.onResolvedCallbacks.forEach(fn => fn());
}
}
let reject = (reason) => {
if(this.status === 'pending'){
this.reason = reason;
this.status = 'rejected';
this.onRejectedCallbacks.forEach(fn => fn());
}
}
try{//执行时可能会发生异常
executor(resolve,reject);
}catch (e){
reject(e);//promise失败了
}
}
then(onFuiFilled,onRejected){
//防止值得穿透
onFuiFilled = typeof onFuiFilled === 'function' ? onFuiFilled : y => y;
onRejected = typeof onRejected === 'function' ? onRejected :err => {throw err;}
let promise2;//作为下一次then方法的promise
if(this.status === 'resolved'){
promise2 = new Promise((resolve,reject) => {
setTimeout(() => {
try{
//成功的逻辑 失败的逻辑
let x = onFuiFilled(this.value);
//看x是不是promise 如果是promise取他的结果 作为promise2成功的的结果
//如果返回一个普通值，作为promise2成功的结果
//resolvePromise可以解析x和promise2之间的关系
//在resolvePromise中传入四个参数，第一个是返回的promise，第二个是返回的结果，第三个和第四个分别是resolve()和reject()的方法。
resolvePromise(promise2,x,resolve,reject)
}catch(e){
reject(e);
}
},0)
});
}
if(this.status === 'rejected'){
promise2 = new Promise((resolve,reject) => {
setTimeout(() => {
try{
let x = onRejected(this.reason);
//在resolvePromise中传入四个参数，第一个是返回的promise，第二个是返回的结果，第三个和第四个分别是resolve()和reject()的方法。
resolvePromise(promise2,x,resolve,reject)
}catch(e){
reject(e);
}
},0)
});
}
//当前既没有完成也没有失败
if(this.status === 'pending'){
promise2 = new Promise((resolve,reject) => {
//把成功的函数一个个存放到成功回调函数数组中
this.onResolvedCallbacks.push( () =>{
setTimeout(() => {
try{
let x = onFuiFilled(this.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
},0)
});
//把失败的函数一个个存放到失败回调函数数组中
this.onRejectedCallbacks.push( ()=>{
setTimeout(() => {
try{
let x = onRejected(this.reason);
resolvePromise(promise2,x,resolve,reject)
}catch(e){
reject(e)
}
},0)
})
})
}
return promise2;//调用then后返回一个新的promise
}
catch (onRejected) {
// catch 方法就是then方法没有成功的简写
return this.then(null, onRejected);
}
}
Promise.all = function (promises) {
//promises是一个promise的数组
return new Promise(function (resolve, reject) {
let arr = []; //arr是最终返回值的结果
let i = 0; // 表示成功了多少次
function processData(index, data) {
arr[index] = data;
if (++i === promises.length) {
resolve(arr);
}
}
for (let i = 0; i < promises.length; i++) {
promises[i].then(function (data) {
processData(i, data)
}, reject)
}
})
}
// 只要有一个promise成功了 就算成功。如果第一个失败了就失败了
Promise.race = function (promises) {
return new Promise((resolve, reject) => {
for (var i = 0; i < promises.length; i++) {
promises[i].then(resolve,reject)
}
})
}
// 生成一个成功的promise
Promise.resolve = function(value){
return new Promise((resolve,reject) => resolve(value);
}
// 生成一个失败的promise
Promise.reject = function(reason){
return new Promise((resolve,reject) => reject(reason));
}
Promise.defer = Promise.deferred = function () {
let dfd = {};
dfd.promise = new Promise( (resolve, reject) => {
dfd.resolve = resolve;
dfd.reject = reject;
});
return dfd
}
module.exports = Promise;