How can I debug my JavaScript code?

Firebug is one of the most popular tools for this purpose.

• Internet Explorer 8 (Developer Tools - F12). Anything else is second rate in Internet Explorer land
• Firefox and Firebug. Hit F12 to display.
• Safari (Show Menu Bar, Preferences -> Advanced -> Show Develop menu bar)
• Google Chrome JavaScript Console (F12 or (Ctrl + Shift + J)). Mostly the same browser as Safari, but Safari is better IMHO.
• Opera (Tools -> Advanced -> Developer Tools)

Start with Firebug and IE Debugger.

Be careful with debuggers in JavaScript though. Every once in a while they will affect the environment just enough to cause some of the errors you are trying to debug.

Examples:

For Internet Explorer, it's generally a gradual slowdown and is some kind of memory leak type deal. After a half hour or so I need to restart. It seems to be fairly regular.

For Firebug, it's probably been more than a year so it may have been an older version. As a result, I don't remember the specifics, but basically the code was not running correctly and after trying to debug it for a while I disabled Firebug and the code worked fine.

Although alert(msg); works in those "I just want to find out whats going on" scenarios... every developer has encountered that case where you end up in a (very large or endless) loop that you can't break out of.

I'd recommend that during development if you want a very in-your-face debug option, use a debug option that lets you break out. (PS Opera, Safari? and Chrome? all have this available in their native dialogs)

//global flag
_debug = true;
function debug(msg){
  if(_debug){
    if(!confirm(msg + '\n\nPress Cancel to stop debugging.')){
      _debug = false;
    }
  }
}

With the above you can get your self into a large loop of popup debugging, where pressing Enter/Ok lets you jump through each message, but pressing Escape/Cancel lets you break out nicely.

I use WebKit's developer menu/console (Safari 4). It is almost identical to Firebug.

console.log() is the new black -- far better than alert().

My first step is always to validate the HTML and to check syntax with JSLint. If you have clean markup and valid JavaScript code then it is time for Firebug or another debugger.

Visual Studio 2008 has some very good JavaScript debugging tools. You can drop a breakpoint in your client side JavaScript code and step through it using the exact same tools as you would the server side code. There is no need to attach to a process or do anything tricky to enable it.

I use a few tools: Fiddler, Firebug, and Visual Studio. I hear Internet Explorer 8 has a good built-in debugger.

I used to use Firebug, until Internet Explorer 8 came out. I'm not a huge fan of Internet Explorer, but after spending some time with the built-in developer tools, which includes a really nice debugger, it seems pointless to use anything else. I have to tip my hat to Microsoft they did a fantastic job on this tool.

You might also check out YUI Logger. All you have to do to use it is include a couple of tags in your HTML. It is a helpful addition to Firebug, which is more or less a must.

I found the new version of Internet Explorer 8 (press F12) is very good to debug JavaScript code.

Of course, Firebug is good if you use Firefox.

Besides using Visual Studio's JavaScript debugger, I wrote my own simple panel that I include to a page. It's simply like the Immediate window of Visual Studio. I can change my variables' values, call my functions, and see variables' values. It simply evaluates the code written in the text field.

I'm using Venkman, a JavaScript debugger for XUL applications.

As with most answers, it really depends: What are you trying to achieve with your debugging? Basic development, fixing performance issues? For basic development, all the previous answers are more than adequate.

For performance testing specifically, I recommend Firebug. Being able to profile which methods are the most expensive in terms of time has been invaluable for a number of projects I have worked on. As client-side libraries become more and more robust, and more responsibility is placed client-side in general, this type of debugging and profiling will only become more useful.

Firebug Console API: http://getfirebug.com/console.html