this指向问题

mounted(){

    let that=this;

    let xhr=new XMLHttpRequest();

    // xhr.responseType="json";

    let user={name:"xiaoliu",password:123456};

  //   xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");

    xhr.open("POST","http://localhost:8080/");

    user=JSON.stringify(user);

    xhr.send(user);

    xhr.onreadystatechange=function(){

      if(xhr.readyState===4){

        // let res=JSON.parse(xhr.response)

        console.log("xhr.response",xhr.response);

        let val = xhr.response;

        that.msg = val;

        // this.$forceUpdate();

        // this.$set(this.msg,xhr.response)

        console.log("this.msg",this.msg)

      }

    }

  },