How can I debug my JavaScript code? [closed]


When I find that I have a problematic code snippet, how should I go about debugging it?

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


All modern browsers come with some form of a built-in JavaScript debugging application. The details of these will be covered on the relevant technologies web pages. My personal preference for debugging JavaScript is Firebug in Firefox. I'm not saying Firebug is better than any other; it depends on your personal preference and you should probably test your site in all browsers anyway (my personal first choice is always Firebug).

I'll cover some of the high-level solutions below, using Firebug as an example:

Firefox

Firefox comes with with its own inbuilt JavaScript debugging tool, but I would recommend you install the Firebug add on. This provides several additional features based on the basic version that are handy. I'm going to only talk about Firebug here.

Once Firebug is installed you can access it like below:

Firstly if you right click on any element you can Inspect Element with Firebug:

Inspect Element in Firebug

Clicking this will open up the Firebug pane at the bottom of the browser:

Firebug pane

Firebug provides several features but the one we're interested in is the script tab. Clicking the script tab opens this window:

Script tab

Obviously, to debug you need to click reload:

JavaScript in the sctipt tab

You can now add breakpoints by clicking the line to the left of the piece of JavaScript code you want to add the breakpoint to:

Adding breakpoints

When your breakpoint is hit, it will look like below:

A breakpoint being hit

You can also add watch variables and generally do everything that you would expect in a modern debugging tool.

Watch varibables

For more information on the various options offered in Firebug, check out the Firebug FAQ.

Chrome

Chrome also has its own in built JavaScript debugging option, which works in a very similar way, right click, inspect element, etc.. Have a look at Chrome Developer Tools. I generally find the stack traces in Chrome better than Firebug.

Internet Explorer

If you're developing in .NET and using Visual Studio using the web development environment you can debug JavaScript code directly by placing breakpoints, etc. Your JavaScript code looks exactly the same as if you were debugging your C# or VB.NET code.

If you don't have this, Internet Explorer also provides all of the tools shown above. Annoyingly, instead of having the right click inspect element features of Chrome or Firefox, you access the developer tools by pressing F12. This question covers most of the points.


  • 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)

There is a debugger keyword in JavaScript to debug the JavaScript code. Put debugger; snippet in your JavaScript code. It will automatically start debugging the JavaScript code at that point.

For example:

Suppose this is your test.js file

function func(){
    //Some stuff
    debugger;  //Debugging is automatically started from here
    //Some stuff
}
func();
  • When the browser runs the web page in developer option with enabled debugger, then it automatically starts debugging from the debugger; point.
  • There should be opened the developer window the browser.

I use old good printf approach (an ancient technique which will work well in any time).

Look to magic %o:

console.log("this is %o, event is %o, host is %s", this, e, location.host);

%o dump clickable and deep-browsable, pretty-printed content of JS object. %s was shown just for a record.

And this:

console.log("%s", new Error().stack);

gives you Java-like stack trace to point of new Error() invocation (including path to file and line number!!).

Both %o and new Error().stack available in Chrome and Firefox.

With such powerful tools you make assumption whats going wrong in your JS, put debug output (don't forget wrap in if statement to reduce amount of data) and verify your assumption. Fix issue or make new assumption or put more debug output to bit problem.

Also for stack traces use:

console.trace();

as say Console

Happy hacking!


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.


In addition to Firebug and browser-native developer extensions JetBrains WebStorm IDE comes with remote debug support for Firefox and Chrome (Extension required) built in.

Also supports:

Options to test this for free are the 30 trial or using an Early Access Version.


If you are using Visual Studio, just put debugger; above the code you want to debug. During execution the control will pause at that place, and you can debug step by step from there on.


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


By pressing F12 web developers can quickly debug JavaScript code without leaving the browser. It is built into every installation of Windows.

In Internet Explorer 11, F12 tools provides debugging tools such as breakpoints, watch and local variable viewing, and a console for messages and immediate code execution.