ES6新特性八：async函数用法实例详解

var fs = require('fs');
//读取文件的方法
var readFile = function (fileName) {
 return new Promise(function (resolve, reject) {
 fs.readFile(fileName, function(error, data) {
  if (error) reject(error);
  resolve(data);
 });
 });
};
var gen = function* () {
 var f1 = yield readFile('/etc/fstab');
 var f2 = yield readFile('/etc/shells');
 console.log(f1.toString());
 console.log(f2.toString());
};
var asyncReadFile = async function () {
 var f1 = await readFile('/etc/fstab');
 var f2 = await readFile('/etc/shells');
 console.log(f1.toString());
 console.log(f2.toString());
};

/**
 * Created by Dason on 2017/3/12.
 */
var mysql = require('mysql');
var pool = mysql.createPool({
 host : '127.0.0.1' ,
 user : 'root' ,
 password : 'root' ,
 database : 'dason',
 multipleStatements: true
});
//查询所有id的方法
var getResult = function () {
 return new Promise(function(resolve,reject){
  pool.getConnection(function(err,connection){
   if(err){
    console.log(err);
    throw err;
   }
   connection.query('SELECT id FROM user',function(err,result){
    if(err){
     //console.log(err);
     throw err;
     //reject(err);
    }
    console.log(result);
    //[{id:1},{id:2},{id:4},{id:5},{id:6},{id:3},{id:7}]
    connection.release();
    resolve(result);
   });
  });
 });
};
//将所有人的name 改为 A
var updateName = function(id) {
 pool.getConnection(function(err,connection){
  if(err){
   console.log(err);
  }
  connection.query("update user set name='A' where id = ?",[id],function(err,result){
   if(err){
    console.log(err);
   }
   connection.release();
  });
 });
};
async function update() {
 /**
  * getResult 是一个异步函数，放在await 后面，
  * 当函数执行的时候，一旦遇到await就会先返回，
  * 等到异步操作完成，将异步结果返回，再接着执行函数体内后面的语句。
  */
 var result = await getResult();
 console.log("result："+JSON.stringify(result));
 //result：[{"id":1},{"id":2},{"id":4},{"id":5},{"id":6},{"id":3},{"id":7}]
 for(const obj of result){
  updateName(obj.id);
 }
}
update();// async函数调用和普通方法一样

async function f() {
 return 'hello world';//会自动调用Promise.resolve('hello world'),将其转化为promise对象
}
f().then(v => console.log(v))
// "hello world"
async function f() {
 throw new Error('出错了');
}
f().then(
 v => console.log(v),
 e => console.log(e)
)
// Error: 出错了

async function main() {
 try {
 var val1 = await firstStep();
 var val2 = await secondStep(val1);
 var val3 = await thirdStep(val1, val2);
 console.log('Final: ', val3);
 }
 catch (err) {
 console.error(err);
 }
}

async function f() {
 await Promise.reject('出错了');//没有return，即不是async函数返回的promise对象，也会被catch方法的回调函数接收到
}
f()
.then(v => console.log(v))
.catch(e => console.log(e))
// 出错了

//耗时写法
let foo = await getFoo();
let bar = await getBar();
//同时触发
// 写法一
let [foo, bar] = await Promise.all([getFoo(), getBar()]);
// 写法二
let fooPromise = getFoo();
let barPromise = getBar();
let foo = await fooPromise;
let bar = await barPromise;