Javascript xmlHttpRequest对象检测与try..catch

if (window.XMLHttpRequest) 
{ 
    xhr = new XMLHttpRequest();
} 
else if (window.ActiveXObject)
{
    xhr = new ActiveXObject(\"Microsoft.XMLHTTP\");
}

try {
   xhr = new ActiveXObject(\"Microsoft.XMLHTTP\");    //IE 
}
catch(e)    // if not IE
{
  xhr = new XMLHttpRequest(); 
}