Promise异步编程

ES5

var f = function (callback) {    console.log(1);    callback && callback.call();}f(function () {    console.log(2)}) //输出1和2

ES6

var f = function (callback) {    console.log(1);    return new Promise(function (resolve, reject) {        resolve()    })}f().then(function () {    console.log(2)}) //输出1和2