ajax示例

//ajax
function retrieveURL(url,nameOfFormToPost) {
    
  //convert the url to a string
  url=url+getFormAsString(nameOfFormToPost);
    
  //Do the AJAX call
  if (window.XMLHttpRequest) { 
     
    // Non-IE browsers
    req = new XMLHttpRequest();
    req.onreadystatechange = processStateChange;
    try {
          req.open("GET", url, true); 
    } catch (e) {
      alert("Server Communication Problem\n"+e);
    }
    req.send(null);
  } else if (window.ActiveXObject) {
    // IE
   
    req = new ActiveXObject("Microsoft.XMLHTTP");
    if (req) {
      req.onreadystatechange=processStateChange;
      req.open("GET", url, true);
      req.send();
    }
  }
}

//取到FOME中的参数
function getFormAsString(formName){
        
  //Setup the return String
  returnString ="";
        
  //Get the form values
  formElements=document.forms[formName].elements;
        
  //loop through the array, building up the url
  //in the format '/strutsaction.do&name=value'
 
  for(var i=formElements.length-1;i>=0; --i ){
        //we escape (encode) each value
        returnString+="&" 
        +escape(formElements[i].name)+"=" 
        +escape(formElements[i].value);
 }
        
 //return the values
 return returnString; 
}

//这个是回调函数
function processStateChange() {
  var info = document.getElementById("alert_message").value;
  if (req.readyState == 4) { // Complete
    if (req.status == 200) { // OK response
        //这里做回调的处理
        var json = eval('(' + req.responseText + ')'); 
        if(json.flag=='false'){
        	alert(json.message+""+info);
        }else if(json.flag=='true'){
        	doQuery();
        }else if(json.flag=='somefalse'){
        	//清空删除列表
    		clearChecked();
        	alert(json.message+""+info);
        	doQuery();
        }
	    /*//Split the text response into Span elements
	    spanElements = 
	        splitTextIntoSpan(req.responseText);
	    
	    //Use these span elements to update the page
	    replaceExistingWithNewHtml(spanElements);
	    */
    } else {
      alert("Problem with server response:\n " 
        + req.statusText);
    }
  }
}

后台返回JSON数据如

：{message:'xxx',flag:'xxx'}

ajax 拿到JSON使用

var json = eval('(' + req.responseText + ')');