Promise 个性是异步编程的一种解决方案，对象代表了将来将要产生的事件，通过Promise，能够防止回调函数的层层嵌套
Promise有两个特色
- 对象的状态代表异步操作后果，一共有三种状态，
pending:进行中
resolved:已胜利
rejected:已失败,
只有异步操作的后果可能决定对象的状态 - 对象状态一旦扭转，就不能再被扭转，且状态只能从pending扭转为resolved以及pending扭转为rejected
Promise对象通过Promise构造函数生成，构造函数接管一个函数为参数，Promise对象建设后就会立刻执行参数函数，此函数以resolve，reject两个函数作为参数，执行resolve函数可使Promise对象状态更改为resolved，执行reject函数可使Promise对象状态更改为rejected，
new Promise((resolve, reject) => {}); new Promise((resolve, reject) => { resolve() }); new Promise((resolve, reject) => { reject() })then()
Promise对象能够通过then()办法来执行resolve(),reject()回调，同时返回一个新Promise对象,其中reject()参数可选
promise.then(resolve, reject) var obj;var resolvePromise = new Promise((resolve, reject) => { var param = {}; obj = param; resolve(param); });须要留神的是resolve和reject并不会终止Promise的执行
new Promise((resolve, reject) => { resolve(); console.log("Promise没有进行执行") });catch()
Promise能够通过catch来执行指定产生谬误时的回调函数同时返回一个新Promise对象
new Promise((resolve, reject) => { reject("产生谬误"); }).catch(error=>{console.log(error)});then办法指定的回调函数产生谬误，也会被catch捕捉
new Promise((resolve, reject) => { resolve("resolve"); }).then(info => { var result = wrong+info }).catch(err=>{ console.log("打印错误信息") console.log(err) })如果Promise曾经resolved，那么之后的谬误将不再捕获
new Promise((resolve, reject) => { resolve("resolve"); var result = wrong }).catch(err=>{ console.log("打印错误信息") console.log(err) })catch办法中还能再次抛出谬误
new Promise((resolve, reject) => { var result = wrong resolve() }).catch(err=>{ console.log("打印错误信息") console.log(err) let obj = {} obj.err.size = 0; }).catch(err=>{ console.log("再一次捕获谬误，打印错误信息") console.log(err) })finally()
promise最初不论状态如何，都会执行finally()
new Promise((resolve, reject) => { var result = wrong resolve() }).catch(err=>{ console.log("打印错误信息") console.log(err) let obj = {} obj.err.size = 0; }).catch(err=>{ console.log("再一次捕获谬误，打印错误信息") console.log(err) }).finally(info=>{ console.log("finally都会执行") })Promise.all()
Promise.all()用于将多个Promise实例包装成一个Promise
Promise.all([new Promise((resolve, reject) => { }), new Promise((resolve, reject) => { }), new Promise((resolve, reject) => { })])- 只有所有的Promise对象的状态为resolved时，才会返回一个resolved状态的Promise，同时值为办法中所有的promise的返回值包装的数组
Promise.all([new Promise((resolve, reject) => { resolve() }), new Promise((resolve, reject) => { resolve() }), new Promise((resolve, reject) => { resolve() })])2.只有办法中有一个Promise被rejected,Promise.all()将返回一个状态为rejected，值为第一个被rejected的Promise对象的值
Promise.all([new Promise((resolve, reject) => { resolve() }), new Promise((resolve, reject) => { reject("第一个被rejected的值") }), new Promise((resolve, reject) => { reject("第二个被rejected的值") })])3 参数中的Promise对象定义了catch办法，被rejected后会触发自生catch(),不会触发Promise.all()的catch()
Promise.all([new Promise((resolve, reject) => { resolve() }), new Promise((resolve, reject) => { reject("被rejected") }).catch(info=>{ console.log("Promise的catch") }), new Promise((resolve, reject) => { resolve() })]).catch(info=>{ console.log("promise.all()的catch") })4参数中的Promise对象没有定义了catch办法，被rejected后会触发Promise.all()的catch()
Promise.all([new Promise((resolve, reject) => { resolve() }), new Promise((resolve, reject) => { reject("被rejected") }), new Promise((resolve, reject) => { resolve() })]).catch(info=>{ console.log("promise.all()的catch") })Promise.race()
Promise.all()用于将多个Promise实例包装成一个Promise,最先扭转状态Promise实例对象的决定Promise.all()返回的实例
Promise.all([new Promise((resolve, reject) => { setTimeout(()=>{resolve("promise1")},1000) }), new Promise((resolve, reject) => { setTimeout(()=>{resolve("promise2")},5000) }), new Promise((resolve, reject) => { setTimeout(()=>{resolve("promise3")},500) })])