如何利用XMLHttpRequest对象进行ajax交互

var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    document.getElementById("myDiv").innerHTML = this.responseText;
  }
};
xmlhttp.open("GET", "index.php", true);
xmlhttp.send();