js promise用法

promise 用法

定义
var promise = new Promise(function(resolve, reject) {
  // ... some code
  if (/* 异步操作成功 */){
    resolve(value);
  } else {
    reject(error);
  }
});
使用
promise.then(function(value) {
  // success
}, function(error) {
  // failure
});
//等价于：
promise.then(function(){
  //success
}).catch(function(){
  //failure
})

es6 promise ajax

定义
const myHttpClient = url => {
  return new Promise((resolve, reject) => {
    let 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);
      } else {
        reject(new Error(this.statusText));
      }
    }
  });
};
使用
myHttpClient('https://www.baidu.com').then(res => {
  console.log(res);
}).catch(error => {
  console.log(error);
});