Javascript: Uncaught TypeError: Cannot call method 'addEventListener' of null

I'm trying to do something fairly simple, but for the reason of me probably not being good enough to search documentation, I can't get this to work.

I have a functioning inline JS that looks like this:

<A title="Wolfram IP Calc" href="javascript:txt=prompt('Enter%20IP%20address,%20e.g.%2010.20.30.40/29','1.2.3.4/5');%20if(txt)%20window.open('http://www.wolframalpha.com/input/?i='+txt);void(O);">Compute!</A>

For various reasons, I'm trying to seperate the JS, and this is where I hit a snag.

I've created the following test page that gives me the error Uncaught TypeError: Cannot call method 'addEventListener' of null:

<HTML> <HEAD profile="http://www.w3.org/2005/10/profile"> <script type="text/javascript">
var compute = document.getElementById('compute');
compute.addEventListener('click', computeThatThing, false);

function computeThatThing() {
    txt=prompt('Enter%20IP%20address,%20e.g.%2010.20.30.40/29','1.2.3.4/5');
    if(txt) {
        window.open('http://www.wolframalpha.com/input/?i='+txt);
    }
}
</script></HEAD>
<BODY>
<A title="Wolfram IP Calc" id="compute" href="javascript:void(O);">Test</A>
</BODY>
</HTML>

The only thing I've been able to find that points to a problem like that is that addEventListener can't work with <A> but should handle <IMG> (which suits me fine as I'm going to pour this on some images), so I tried adding the following to no avail:

<img id="compute" src="http://products.wolframalpha.com/images/products/products-wa.png" />

Thanks in advance for pointing out what I'm doing wrong. It is probably glaringly obvious, but I have close to zero experience with JS and I have gone mostly by cargo culting when I've needed it until now.

Answers:

Answer

Your code is in the <head> => runs before the elements are rendered, so document.getElementById('compute'); returns null, as MDN promise...

element = document.getElementById(id);
element is a reference to an Element object, or null if an element with the specified ID is not in the document.

MDN

Solutions:

  1. Put the scripts in the bottom of the page.
  2. Call the attach code in the load event.
  3. Use jQuery library and it's DOM ready event.

What is the jQuery ready event and why is it needed?
(why no just JavaScript's load event):

While JavaScript provides the load event for executing code when a page is rendered, this event does not get triggered until all assets such as images have been completely received. In most cases, the script can be run as soon as the DOM hierarchy has been fully constructed. The handler passed to .ready() is guaranteed to be executed after the DOM is ready, so this is usually the best place to attach all other event handlers...
...

ready docs

Answer

Move script tag at the end of BODY instead of HEAD because in current code when the script is computed html element doesn't exist in document.

Since you don't want to you jquery. Use window.onload or document.onload to execute the entire piece of code that you have in current script tag. window.onload vs document.onload

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