简易版手写ajax

//参数：method,url,header,data,success,faild
function ajax(obj){
  let promise = new Promise(function(resolve,reject){
    let  request = new XMLHttpRequest()
    request.open(obj.method,obj.url)
    request.setRequestHeader('Content-Type', obj.header || "application/x-www-form-urlencoded")
    request.onreadystatechage = () =>{
      if(request.readystate ===4){
        if(status === 200){
          resolve(request.responseText)
        }
        else{
          reject(new Error(request.statusText))
        }
      }
    }
    request.send(obj.data || {})
  })
  promise.then(obj.success,obj.faild)
}