Promise介绍和使用

Promise状态

Promise 对象代表一个异步操作，不受外界影响，它有三种状态:

1. pending (进行中)

2. resolved (完成)

3. rejected (失败)

Promise的优势

1. 解决回调地狱

   // 回调地狱
   async(1, function (value) {
     async(value, function (value) {
       async(value, function (value) {
         async(value, function (value) {
           ...
         })
       })
     })
   })

   使用 Promise :

   // setTimeout 多余的参数会作为入参传递给第一个参数方法
   let firstAsync = function (input) {
     return new Promise(function (resolve, reject) {
       setTimeout(resolve, 500, input * input)
     })
   }
   let secondAsync = function (input) {
     return new Promise(function (resolve, reject) {
       setTimeout(resolve, 500, input + input)
     })
   }
   let start = new Promise(function (resolve, reject) {
     resolve(123)
   })
   start.then(firstAsync).then(secondAsync).then(function (res) {
     console.log('最终结果: ' + res)
   })

2. 进行错误捕获

   // 注意: reject入参需要是一个Error对象，否则报错
   function ajax(method, url, data) {
     let request = new XMLHttpRequest()
     return new Promise(function (resolve, reject) {
       request.onreadystatechange = function () {
         if (request.readyState === 4) {
           if (request.status === 200) {
             resolve(request.responseText)
           } else {
             reject(new Error(request.status))
           }
         }
       }
       request.open(method, url)
       request.send(data)
     })
   }
   
   let start = ajax('GET', '/api/test')
   start.then(function (res) {
     console.log(res)
   }).catch(function (error) {
     console.log(error)
   })

其他用法

1. Promise.all() : 并行执行,所有异步操作完成后才执行回调

   let p1 = new Promise(function (resolve, reject) {
     setTimeout(resolve, 1000, '异步1')
   })
   let p2 = new Promise(function (resolve, reject) {
     setTimeout(resolve, 3000, '异步2')
   })
   Promise.all([p1, p2]).then(function (res) {
     console.log(res)
   })

2. Promise.race() : 仅获得先返回的结果，其余异步任会执行,但结果丢弃

   let p1 = new Promise(function (resolve, reject) {
     setTimeout(resolve, 1000, '异步1')
   })
   let p2 = new Promise(function (resolve, reject) {
     setTimeout(resolve, 3000, '异步2')
   })
   Promise.race([p1, p2]).then(function (res) {
     console.log(res)
   })

参考

1. Promise-廖雪峰
2. js promise看这篇就够了
3. JS - Promise使用详解