Promise并行执行

Promise.all() 等待所有任务结束

function ajax (url) {
  return new Promise(function(resolve, reject) {
    var xhr = new XMLHttpRequest()
    xhr.open('GET', url)
    xhr.responseType = 'json'
    // readystate = 4
    xhr.onload = function () {
      if (this.status === 200) {
        resolve(this.response)
      } else {
        reject(this.statusText)
      }
    }
    xhr.send()
  })
}

var promise = Promise.all([
  ajax('/api/users.json'),
  ajax('/api/posts.json')
])
promise.then(function(value) {
  console.log(value)
}).catch(function(error) {
  console.log(error)
})

ajax('/api/urls.json')
  .then(value => {
    const urls = Object.values(value)
    const tasks = urls.map(url => ajax(url))
    return Promise.all(tasks)
  })
  .then(values => {
    console.log(values)
  })

Promise.race() 只会等待第一个结束的任务
举例请求超时的处理方式

const request = ajax('/api/posts.json')
const timeout = new Promise((resolve, reject) => {
  setTimeout(() => reject(new Error('timeout')), 500)
})

Promise.race([
  request,
  timeout
]).then(value => {
  console.log(value)
})