JavaScript 函数惰性载入的实现及其优点介绍

最近看JavaScript高级程序设计，大有收获，接下来几天写一下读书笔记。之前写了一篇Ajax初步理解的随笔，里面有个函数用来创建XmlHttpRequest对象,浏览器兼容性原因，写出的代码通过大量if判断或者try，catch语句将函数引导到正确代码处。

复制代码 代码如下:

<script type="text/javascript">
function createXHR(){
var xhr = null;
try {
// Firefox, Opera 8.0+, Safari，IE7+
xhr = new XMLHttpRequest();
}
catch (e) {
// Internet Explorer
try {
xhr = new ActiveXObject("Msxml2.XMLHTTP");
}
catch (e) {
try {
xhr = new ActiveXObject("Microsoft.XMLHTTP");
}
catch (e) {
xhr = null;
}
}
}
return xhr;
}
</script>

复制代码 代码如下:

function createXHR(){
var xhr=null;
if(typeof XMLHttpRequest !='undefined'){
xhr = new XMLHttpRequest();
createXHR=function(){
return new XMLHttpRequest();
}
}else{
try {
xhr = new ActiveXObject("Msxml2.XMLHTTP");
createXHR=function(){
return new ActiveXObject("Msxml2.XMLHTTP");
}
}
catch (e) {
try {
xhr = new ActiveXObject("Microsoft.XMLHTTP");
createXHR=function(){
return new ActiveXObject("Microsoft.XMLHTTP");
}
}
catch (e) {
createXHR=function(){
return null;
}
}
}
}
return xhr;
}