In JavaScript, frames are treated just like windows. Just as you can tweak the contents of one window using JavaScript inside a different window, you can change the contents of one frame using JavaScript in another frame.

Here's a straightforward example of using frames with JavaScript. I'll break it down line by line.

The first thing we need to look at is the frameset. Here it is:

<frameset rows="25%,*">
<frame src="frames_example_controls.html" name="control_frame">
<frame src="blank.html" name="target_frame">
</frameset>

This is just like any ordinary frameset. The important thing to note and remember is that the frames in the frameset are named. The first frame, called control_frame, holds the HTML page that contains the JavaScript. The second frame, as you can see from the src="blank.html", contains nothing.

The second thing to look at is the contents of the control_frame. To keep things simple, it only has one interesting line:

<a href="#" onClick="top.target_frame.document.writeln('Monkey do!<br>');">Monkey see</a>

This line tries to write the words "Monkey do!<br>" into the bottom frame. But before JavaScript can write to the frame, it has to know what frame we're talking about. The expression top.target_frame, which you'll see in the onClick, tells JavaScript to look at the top-most window, which is the browser itself, and find the thing called target_frame inside that window. Because we called the bottom frame "target_frame" when defining the frameset, JavaScript will know that target_frame means the bottom frame.

Once we've told JavaScript which frame we're talking about, we can treat the frame just like a window. To write the word "howdy" to a window called greeting_window, we'd do this:

greeting_window.document.writeln("howdy");

This tells JavaScript to find the window named greeting_window, then find the document of that window, and then write the word "howdy" into that document. Yes, if you haven't figured it out yet, writeln() is a method of the document object, which is why we've had to write document.writeln('blah blah) instead of just writeln('blah blah').

Because frames are treated just like windows, we can do a similar thing:

top.target_frame.document.writeln("Monkey do!");

Now, let's learn more about windows and frames.