AJAX

function loadXMLDoc(){
    var xmlhttp;
    if (window.XMLHttpRequest) {
      //  IE+,  Firefox,  Chrome,  Opera,  Safari
      xmlhttp = new XMLHttpRequest();
    } else {
      xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange = function(){
       if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
         //你的执行代码 
       }
    }
    xmlhttp.open("GET", url, true);
    xmlhttp.send();
}