SOURCE

xhr= new XMLHttpRequest();
xhr.open('GET','http://cdn.jsrun.top/css/img/logo@2x.png',true);//第三个参数指定异步/同步
xhr.send();
xhr.onreadystatechange=function(){
  console.log(xhr.readyState,xhr.status)
  console.log(xhr.responseText,xhr.responseXML,xhr)
}
xhr.onload=function(){
  console.log('onload',xhr.readyState)
}
xhr.onloadend=()=>console.log('loadend',xhr.readyState)
xhr.onprogress=()=>console.log('progress',xhr.readyState)

xhr.onloadstart=()=>console.log('start',xhr.readyState)