同步可以改变外部定义的变量值,异步可以提高加载效率
a. 说明:
b. 原代码:
<script> var a = 0 function loadDoc(file,async=true){ if(window.XMLHttpRequest){ // code for IE7+,Firefox,Chrome,Opera,Safari xmlhttp=new XMLHttpRequest(); }else{ // code for IE6,IE5 xmlhttp=new ActiveXObject("Microsoft.XMLHTTP"); } if(async === true){ xmlhttp.onreadystatechange = function(){ if(xmlhttp.readyState < 4){ // 加载中 }else if (xmlhttp.readyState == 4 && xmlhttp.status == 200){ // 成功 xmlDoc=xmlhttp.responseText; a = 1 return xmlDoc }else{ // 失败 xmlhttp.abort() return } } } xmlhttp.open("GET",file,async); xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded"); xmlhttp.send(); if(async === false){ xmlDoc=xmlhttp.responseText; a = 1 return xmlDoc } } var ret = loadDoc(‘http://localhost/test.PHP‘,true) console.log(a) var ret = loadDoc(‘http://localhost/test.PHP‘,false) console.log(a) </script>