html - How to make Event Listeners work with external Javascript File?

I have these simple files -

document.getElementById("btn2").addEventListener("click",function(){
	alert("Did it work?");
});

which work correctly here and also on jsFiddle, but when I run this on my computer locally, the EventListener won't work! I've been trying to fix this all day and no success. What is the problem? I just cannot seem to figure it out!