一、Promise 是一个异步操作返回的对象，用来传递异步操作的消息。
Promise 介绍和使用详见：MDN对Promise的介绍
一句话概括就是：Promise对象用于表示一个异步操作的最终完成 (或失败)及其结果值
二、根据自己对 Promise 的理解，实现一个Promise :
Promise 有三种状态：Pending 初始态； Fulfilled 成功态； Rejected 失败态。
let Promise = (executor) => {
let that= this;
that.status = 'pending'; //等待态
that.value = undefined; //成功的返回值
that.reason = undefined; //失败的原因
function resolve(value){
if(that.status === 'pending'){
that.status = 'resolved';
that.value = value;
}
}
function reject(reason) {
if(that.status === 'pending') {
that.status = 'rejected';
that.reason = reason;
}
}
try{
executor(resolve, reject);
}catch(e){
reject(e);// 捕获时发生异常，就直接失败
}
}
//onFufiled 成功的回调
//onRejected 失败的回调
//用来指定 Promise 对象的状态改变时要执行的操作
Promise.prototype.then = (onFufiled, onRejected)=> {
let that= this;
if(that.status === 'resolved'){
onFufiled(that.value);
}
if(that.status === 'rejected'){
onRejected(that.reason);
}
}
export default { Promise };
测试样例
import Promise from './Promise'
let promise = new Promise((resolve, reject) => {
resolve("成功回调");
})
promise.then((res) => {
console.log('res:', res);
},(err) => {
console.log('err:', err);
})
