Why do you see colons while calling a javascript function in html sometimes?

Question

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

Like onclick=\"javascript:functionname();\"

The same function works wit

Answer 1

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

Anchor

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)