The Book of JavaScript, 2nd Edition

More About Functions

Whereas variables store information, functions process that information.

All functions take the form functionName(). Sometimes there's something in the parentheses and sometimes there isn't. You've already seen one of JavaScript's many built-in functions, window.document.write(), which writes whatever lies between the parentheses to the web page. Before diving into the date functions that you'll need to write the date to your web page, I'll talk about two interesting functions, just so you get the hang of how functions work.

alert()

One handy function is alert(), which puts a string into a little announcement box (also called an alert box). Figure 2-7 demonstrates how to call an alert(), and Figure 2-8 shows what the alert box ...