Defer Java Script Confusion

About the above topic:
Where do I keep my external JS file? In the theme directory?
Then what about the code to call the external JS file as shown below:

[ Moderator note: code fixed. Please wrap code in the backtick character or use the code button. ]

<em><script type="text/javascript">
function deferJSWordPress() {
var element = document.createElement("script");
element.src = "defer.js";
document.body.appendChild(element);
}
if (window.addEventListener)
window.addEventListener("load", deferJSWordPress, false);
else if (window.attachEvent)
window.attachEvent("onload", deferJSWordPress);
else window.onload = deferJSWordPress;
</script>
</body>
</html></em>

If my javascript external file is called defer.js, then I just add the word “defer.js” as shown above? Or do I have to indicate its path way. Meaning like this: wp-content/theme/name of theme/defer.js?