ajax原理

XMLHttpRequest对象

function ajaxJson(url,type){
   var promise = new Promise(function(resolve,reject){
      var handle = function(){
        if(this.readyState !== 4){
        return;
        }
        if(this.status == 200){
            resolve(this.response)
        }else{
            reject(new Error(this.statusText));
        }
    }
  var client = new XMLHttpRequest();
  client.onreadyStatusChange = handle;
  client.open(url,type);
  client.responseType = "json";
  client.send();
})
  return promise;
}