Question

A lot of the time I see people calling javascript functions using colon (:).

Like onclick="javascript:functionname();"

The same function works without javascript: and I’m curious to know when to use javascript: and when not to.

Any suggestions are appreciated.

Answer 1

javascript: prefix is extremely important when you put code to anchor href attribute:

<a href="javascript:func();">Anchor</a>

Whereas in inline event attributes (like onclick, onsubmit, onmouseover, etc.) javascript: prefix is not important.

However, you should note that both approaches given here are not good to implement and you should use alternative ways (e.g as @Paul S. stated in the comments)