In an element's inline event handler, like onclick, onchange, onsubmit, etc., you do not need the javascript: prefix - the reason you often see it is because people confuse it with the href syntax that I explain below. It doesn't cause an error - I believe it is interpreted as a label - but it is unnecessary.

It doesn't matter whether you want to call a function or run a simple JS statement, either way don't include javascript: - that is, all three of these are valid:

onclick=doSomething('some val');

onclick=return false;

onclick=doSomething(); doSomethingElse(); return false;

If you are using inline event attributes don't use the javascript: prefix.

(I say if you are using inline event attributes because this practice is really outdated: it is better to assign the event handlers using JS in a script block.)

You only need the javascript: prefix when you want to run JavaScript from an <a> element's href attribute like this:

<a href=javascript: someFunc();>Whatever</a>

That is because the browser normally expects the href to contain a URI, so the javascript: prefix tells it to expect JS code instead. However, I don't recommending doing that because the page won't work for people who have JS disabled. Better to include an href that directs the user to a page telling them to enable JS, and include an onclick to do the JS functionality:

<a href=/enableJS.html onclick=doSomething(); return false;>Whatever</a>

That way the link does something useful for users whether they have JS enabled or not. The return false at the end of the click handler prevents the default behaviour for a click (which would be to navigate to the specified URL).