$.ajax(url1, function(data1){
 // do something...
 $.ajax(url2, function(data2){
 // do something...
 $.ajax(url3, function(data3){
 // do something...
 done(data3); // 返回数据
 })
 });
});

var promise = function(isReady){
 return new Promise(function(resolve, reject){
 // do somthing, maybe async
 if (isReady){
  return resolve('hello world');
 } else {
  return reject('failure');
 }
 });
}
//Promise实例生成以后，可以用then方法分别指定Resolved状态和Reject状态的回调函数。
promise(true).then(function(value){
 // success,这里是resolve的回调函数
 console.log(value); //hello world
}, function(err){
 // failure，这里是reject的回调函数
 console.log(err)
})

makePromise1()
.then(function(value){
 console.log(value);
 return makePromise2();
})
.then(function(value){
 console.log(value);
 return makePromise3();
})
.then(function(value){
 console.log(value);
});
function makePromise1(){
 var p = new Promise(function(resolve, reject){
 //异步操作
 setTimeout(function(){
  console.log('异步任务1');
  resolve('异步任务1传过来的值');
 }, 2000);
 });
 return p;  
}
function makePromise2(){
 var p = new Promise(function(resolve, reject){
 //异步操作
 setTimeout(function(){
  console.log('异步任务2');
  resolve('异步任务2传过来的值');
 }, 2000);
 });
 return p;  
}
function makePromise3(){
 var p = new Promise(function(resolve, reject){
 //异步操作
 setTimeout(function(){
  console.log('异步任务3');
  resolve('异步任务3传过来的值');
 }, 2000);
 });
 return p;  
}

异步任务1
异步任务1传过来的值
异步任务2
异步任务2传过来的值
异步任务3
异步任务3传过来的值

var promise = function(isReady){
 return new Promise(function(resolve, reject){
 // do somthing, maybe async
 if (isReady){
  return resolve('hello world');
 } else {
  return reject('failure');
 }
 });
}
promise(true)
.then(function(value){
 console.log('resolved');
 console.log(value);
 console.log(haha); //此处的haha未定义
})
.catch(function(error){
 console.log('rejected');
 console.log(error);
});

resolved
hello world
rejected
ReferenceError: haha is not defined(…)

var p1 = new Promise(function (resolve) {
 setTimeout(function () {
 resolve("第一个promise");
 }, 3000);
});
var p2 = new Promise(function (resolve) {
 setTimeout(function () {
 resolve("第二个promise");
 }, 1000);
});
Promise.all([p1, p2]).then(function (result) {
 console.log(result); // ["第一个promise", "第二个promise"]
});

var p1 = new Promise(function (resolve) {
 setTimeout(function () {
 console.log(1);
 resolve("第一个promise");
 }, 3000);
});
var p2 = new Promise(function (resolve) {
 setTimeout(function () {
 console.log(2);
 resolve("第二个promise");
 }, 1000);
});
Promise.race([p1, p2]).then(function (result) {
 console.log(result); 
});
// 结果：
// 2
// 第二个promise
// 1