responseText 属性

点击按钮试一下

<script>
function loadXMLDocChange()
{
var xmlhttp;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("loadXMLDocChange").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","/demo/ajax-get-change.php",true);
xmlhttp.send();
}
</script>
<div id="loadXMLDocChange"><code>点击按钮试一下</code></div></br>
<button type="button" onclick="loadXMLDocChange()">通过 AJAX 改变内容</button>
//
ajax-get-change.php
<?php
    echo "<code>你把原来的内容改变了。</code>";
?>

参考自 W3C