generator
async和await是generator的语法糖，想要搞清楚async和await必须晓得generator;那么咱们先来看看generator函数；
function * getAge() { yield 1; yield 2;}let gen = getAge();console.log(gen.next());//{ value: 1, done: false }console.log(gen.next());//{ value: 2, done: false }console.log(gen.next());//{ value: undefined, done: true }generator和一般函数的区别：
1.generator申明函数之前要加*
2.函数中有yield，能够管制函数的运行
generator传参问题
function * getAge() { let a = yield 1; console.log(a);//444 yield 2;}let gen = getAge();console.log(gen.next());console.log(gen.next(444));generator传参能够将参数传在next函数中，给a赋值,然而第一个next()函数永远是undefined;如下
function * getAge(xxx) { let a = yield 1; console.log(a);//undefined yield 2;}let gen = getAge();console.log(gen.next(444));console.log(gen.next());generator函数运行过程
generator实现读取异步函数
当初有一个name和age的txt文件，咱们通过name.txt读取age.txt里的内容;我这里的name中写的是age.txt，age.txt中写的是18;
let {promisify} = require('util');let fs = require('fs');let read = promisify(fs.readFile);function * getAge() { let name = yield read(`${__dirname}name.txt`, 'utf-8'); let age = yield read(`${__dirname}${name}`, 'utf-8');}let it = getAge();let {value} = it.next();//返回的是读取name.txt的promise函数value.then(data => { let {value} = it.next(data)//将读取name文件的内容作为参数赋值给getAge函数中的name,同时这里的value是读取age.txt的promise函数 value.then(data => { console.log(data)//18 })});async和await
下面层层嵌套看着很乱尤其是嵌套多的状况下，那么咱们应用async和await实现一下这里只是将generator函数中的*换成了async，将yield换成了await
async function getAge1() { let name = await read(`${__dirname}name.txt`, 'utf-8'); let age = await read(`${__dirname}${name}`, 'utf-8'); console.log(age);}getAge1()咱们将下面函数进行babel本义成es2015，就是将generaotr+promise联合起来实现了async和await函数
1 将async函数转换成generator
2.第一次执行next传参为undefined，下面咱们也提到了第一次next的值是undefined
3.将上一次的返回值传递给上面的next函数，递归实现找打最初即可
let {promisify} = require('util');let fs = require('fs');let read = promisify(fs.readFile);function asyncGeneratorStep(gen, resolve, reject, _next, key, arg) { try { var info = gen[key](arg); var value = info.value; } catch (error) { reject(error); return; } if (info.done) { resolve(value); } else { //为了避免value是个一般值（不是promise）,应用Promise.resolve包一层 Promise.resolve(value).then(_next); }}function _asyncToGenerator(fn) { return function () { return new Promise(function (resolve, reject) { var gen = fn(); function _next(value) { asyncGeneratorStep(gen, resolve, reject, _next, "next", value); } _next(undefined); }); };}function getAge1() { return _getAge();}function _getAge() { _getAge = _asyncToGenerator(function* () { let name = yield read(`${__dirname}name.txt`, 'utf-8'); let age = yield read(`${__dirname}${name}`, 'utf-8'); }); return _getAge();}getAge1(); //让函数执行