(new Promise(f1)).then(f2);

promise.then(
 console.log,
 console.error
);

promise
 .then(step1)
 .then(step2)
 .then(step3)
 .then(
  console.log,
  console.error
 );

var promise = new Promise(function(resolve, reject) { // promise的构造函数，Promise构造函数接受一个函数作为参数，该函数的两个参数分别是resolve和reject
 // 异步操作的代码
 if (/* 异步操作成功 */){
  resolve(value);//将异步的操作结果作为参数传递出去
 } else {
  reject(error);
 }
});

var getJSON = function(url) {
 var promise = new Promise(function(resolve, reject){
  var client = new XMLHttpRequest();
  client.open("GET", url);
  client.onreadystatechange = handler;
  client.responseType = "json";
  client.setRequestHeader("Accept", "application/json");
  client.send();
  function handler() {
   if (this.readyState !== 4) {
    return;
   }
   if (this.status === 200) {
    resolve(this.response);//会将参数传递给回调函数
   } else {
    reject(new Error(this.statusText));
   }
  };
 });
 return promise;
};
getJSON("/posts.json").then(function(json) {
 console.log('Contents: ' + json);
}, function(error) {
 console.error('出错了', error);
});

var p1 = new Promise(function (resolve, reject) {
 // ...
});
var p2 = new Promise(function (resolve, reject) {
 // ...
 resolve(p1);
})

promise.then(function(posts) {
 // ...
}).catch(function(error) {
 // 处理 getJSON 和 前一个回调函数运行时发生的错误
 console.log('发生错误！', error);
});

var promise = new Promise(function(resolve, reject) {
 throw new Error('test');
});
promise.catch(function(error) {
 console.log(error);
});
// Error: test

// 写法一
var promise = new Promise(function(resolve, reject) {
 try {
  throw new Error('test');
 } catch(e) {
  reject(e);
 }
});
promise.catch(function(error) {
 console.log(error);
});
// 写法二
var promise = new Promise(function(resolve, reject) {
 reject(new Error('test'));//reject方法的作用，等同于抛出错误
});
promise.catch(function(error) {
 console.log(error);
});

var promise = new Promise(function(resolve, reject) {
 resolve('ok');
 throw new Error('test');
});
promise
 .then(function(value) { console.log(value) })
 .catch(function(error) { console.log(error) });
// ok

var p = Promise.all(promises).then(function (posts) {
 // ...
}).catch(function(reason){
 // ...
});

Promise.resolve('foo')
// 等价于
new Promise(resolve => resolve('foo'))

let thenable = {
 then: function(resolve, reject) {
  resolve(42);
 }
};
let p1 = Promise.resolve(thenable);//p1为一个状态已经是resolved 的promise对象.
p1.then(function(value) {
 console.log(value); // 42
});

var p = Promise.resolve('Hello');//Hello 会传递给 p 的resolve()
p.then(function (s){
 console.log(s)
});
// Hello