javaScript如何生成xmlhttp

function ajaxFunction(){ 
var xmlHttp; 
try { // Firefox, Opera 8.0+, Safari 
xmlHttp = new XMLHttpRequest(); 
} 
catch (e) { 
try {// Internet Explorer 
xmlHttp = new ActiveXObject("Msxml2.XMLHTTP"); 
} 
catch (e) { 
try { 
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP"); 
} 
catch (e) { 
} 
} 
} 

return xmlHttp; 
}