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1¡¢  ÊäÈëÓò£º
»áÔ±µÄÓÐЧÐÔ¼ì²é£º
     <input type="text" name="username">&nbsp;&nbsp;<input type="button" value="¼ì²é" onclick="return check();">
2¡¢  JavaScript½Å±¾£º
var xmlHttp;
    //´´½¨XMLHttpRequest¶ÔÏó
    function createXMLHttpRequest() {
       if(window.ActiveXObject) {
           xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
       } else if (window.XMLHttpRequest) {
           xmlHttp = new XMLHttpReuqest();
       }
    //×¢²áÃûÓÐЧÐÔ¼ì²é
    function check() {
       var username = document.getElementById('username').value;
       if(username=="") {
           alert("Óû§Ãû²»ÄÜΪ¿Õ£¡");
           return;
       }
       createXMLHttpRequest();
       xmlHttp.onreadystatechange = processCheck;
       xmlHttp.open("GET", "CheckUser?username=" + username);
       xmlHttp.send(null);
    }
      
    //°ó¶¨µÄÓÐЧÐÔ¼ì²éº¯Êý
    function processCheck() {
       var result;
       if(xmlHttp.readyState == 4) { //Èç¹ûÏìÓ¦Íê³É
           if(xmlHttp.status == 200) { //Èç¹û·µ»Ø³É¹¦
              result = xmlHttp.responseText;
      &