1. Generator生成器函数: 
      1. function * foo () {
      2. console.log('start')
      3. try {
      4.  const res = yield 'foo' // 使用yield关键字向外返回一个值 暂停generator函数的执行
      5.    console.log(res)
      6. }catch (e){
      7.  console.log(e)
      8. }
      9. }
      10. let generator = foo()
      11. const result = generator.next()
      12. generator.next('bar') // 作为yield这个语句的返回值
      13. generator.throw('err')
    // generator + Promise
  function ajax (url) {
    return new Promise(function (resolve, reject) {
      let xhr = new XMLHttpRequest()
      xhr.open('GET', url)
      xhr.responseType = 'json'
      xhr.onload = function () {
        if (this.status + '' === '200') {
          resolve(this.response)
          console.log(this.response)
        } else {
          reject(new Error(this.responseText))
          console.log(this.responseText)
        }
      }
      xhr.send()
    })
  }
function * main () {
  const users = yield ajax('/ajaxTest.js')
  const post = yield ajax('/ajaxPost.js')
}
let g = main()
const result = g.next()
if (result.done) return
result.then(data => {
  const result1 = g.next(data)
  if (resul1.done) return
  result1.value.then(res => {
    g.next(res)
  })
})