const promise = new Promise(function(resolve, reject) {
 //resolve （data）  成功
 //reject  （error ）失败
});

promise.then(function(value) {
 // success
}, function(error) {
 // err
});

function fun(ms){
 return new promise((res,rej) => {
 setTimeout(res,ms)
 })
}
fun(100).then((val) => {
 console.log(val)
})

function loadImageAsync(url) {
 return new Promise(function(resolve, reject) {
  const image = new Image();
  image.onload = function() {
   resolve(image);
  };
  image.onerror = function() {
   reject(new Error('Could not load image at ' + url));
  };
  image.src = url;
 });
}

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

promise
.then(result => {···})
.catch(error => {···})
.finally(() => {···});

//有时需要将现有对象转为 Promise 对象，Promise.resolve方法就起到这个作用。
const jsPromise = Promise.resolve($.ajax('/whatever.json'));

Promise.resolve('foo')
// 等价于
new Promise(resolve => resolve('foo'))
//Promise.resolve方法的参数分成四种情况。