The debugger we like is the Web Developer toolbar by Chris Pederick. It's a Firefox extension. You can get it here:
Once installed, you'll see a toolbar like this one:
As you can see, there's lots of options for budding web developers.
When you click the red X you'll see a console pop up. The console will tell you some information about the error:
Although the message itself can be puzzling if you're a beginner, it at least tells you which line the error is on.
You can also type something in the Code text box and click the evaluate button:
If there's an error you'll then see this:
If you want to try this, type a lowercase "a" and click evaluate again. You'll see a message box appear. The code has run successfully.
Internet Explorer 9 has a sophisticated Jscript debugger built into it. (Jscript
is tricky to find, though. First, press the left ALT key on your keyboard to
see the File, Edit, View menu at the top. Now click Tools > Internet Options.
From the Internet Options dialogue box click the Advanced tab. Scroll
down and find the entry for Disable Script Debugging. Uncheck this entry
if it has a check mark next to it. Click the Apply button at the bottom
of Internet Options. Reload your web page and you'll see a message at the bottom
about ActiveX controls. Say yes to this, and yes on the Web Page Error dialogue
box. After all that, you should see the following:
The debugger is not the easiest thing in the world to understand, but at least you get a nice yellow background and pointer for your errors!