How to call javascript function specified in a child

I’m working on a ancient project which is based on and tags.

Here’s the common HTML structure of the parent page (index.html):

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>  
</head>
<frameset rows="75px,*" frameborder="0">
    <frame name="frame1" src="frames/frame1.html">
    <frameset class="child_frameset">
        <frame name="frame3" src="frames/frame3.html">
    </frameset>
</frameset>
</html>

Here how I specify the javascript function on the child frame (frame3.html):

<!DOCTYPE html>  
<html lang="en">  
<head>
    <script type="text/javascript">
        function helloWorld(){
            alert('hello world!');
        }
    </script>   
</head>
<body>
    <h1>I'm frame 3</h1>
</body>
</html>

My task is to call helloWorld() function. How do I do that?
Thanks in advance!

Answer

You can do it like this:

document.getElementById('frame3').contentWindow.helloWorld();