new Promise( function(resolve, reject) {...} /* executor */ );

promise.then(function(result) {
 console.log(result); // "Stuff worked!"
}, function(err) {
 console.log(err); // Error:"It broke"
});

const myFirstPromise = new Promise((resolve, reject) => {
 // "failure reason"); // rejected
});

function myAsyncFunction(url) {
 return new Promise((resolve, reject) => {
 const xhr = new XMLHttpRequest();
 xhr.open("GET", url);
 xhr.onload = () => resolve(xhr.responseText);
 xhr.onerror = () => reject(xhr.statusText);
 xhr.send();
 });
};

function get(url) {
 // 返回一个 promise 对象.
 return new Promise(function(resolve, reject) {
 // 创建一个 XML 对象
 var req = new XMLHttpRequest();
 req.open('GET', url);

 req.onload = function() {
  
  if (req.status == 200) {
  // 请求 200的时候处理 response 
  resolve(req.response);
  }
  else {
  // 处理请求错误信息
  reject(Error(req.statusText));
  }
 };

 // 处理网络错误信息
 req.onerror = function() {
  reject(Error("Network Error"));
 };

 // 发送请求
 req.send();
 });
}

get('story.json').then(function(response) {
 console.log("Success!", response);
}, function(error) {
 console.error("Failed!", error);
})

function imgLoad(url) {
 //创建一个图片上传的 Promise() constructor;
 return new Promise(function(resolve, reject) {
  
  var request = new XMLHttpRequest();
  request.open('GET', url);
  request.responseType = 'blob';
  
  request.onload = function() {
  if (request.status === 200) {
  
   resolve(request.response);
  } else {
  
   reject(Error('Image didn\'t load successfully; error code:' + request.statusText));
  }
  };
  request.onerror = function() {
  
   reject(Error('There was a network error.'));
  };
  
  request.send();
 });
 }

 // 挂载到 body 上面去
 var body = document.querySelector('body');
 var myImage = new Image();
 
 // 使用
 imgLoad('myLittleVader.jpg').then(function(response) {
 
 //第一步 处理 resolve() method.
 var imageURL = window.URL.createObjectURL(response);
 myImage.src = imageURL;
 body.appendChild(myImage);
 
 }, function(Error) {
 
 // 处理错误
 console.log(Error);
 });