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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:
Clicking this will open up the Firebug pane at the bottom of the browser:
Firebug provides several features but the one we're interested in is the script tab. Clicking the script tab opens this window:
Obviously, to debug you need to click reload:
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:
When your breakpoint is hit, it will look like below:
You can also add watch variables and generally do everything that you would expect in a modern debugging tool.
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:
coffescript: how to debug coffeescript in node.js with webstorm 6 source maps
node.js
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.

Development Mode requires the GWT Developer Plugin

My project using GWT. I haved installed GWT plugin for my Chrome Browser before . I am using the newest version of chrome .When run the project, Chrome browser alway announce
Development Mode requires the GWT Developer Plugin
I have attached the image bellow
But when I click to the link. Chrome browser go to page of plugin in the Chrome web Store. But it announce me that my chrome browser haved installed this plugin. I have attached the screenshot bellow
How can I make GWT plugin work with my project ?
Thank for any help

From GWT Website ,http://www.gwtproject.org/release-notes.html#Release_Notes_2_6_1
GWT Development Mode will no longer be available for Chrome sometime in 2014, so we improved alternate ways of debugging. There are improvements to Super Dev Mode, asserts, console logging, and error messages.
This is because of newer chrome version.Use Super Dev Mode
Other Solution is you can enable NPAPI which was disable in chrome 42 version follow the below steps
1.Open New Tab and Enter chrome://flags/#enable-npapi
2.Enable this Enable NPAPI Mac, Windows
3.Then bottom of the page click "Relunch" button.

I had the same issue this morning.
Switching to super dev mode:
select your project in the Project Explorer -> Debug as -> Web Application(Super Dev Mode)
Console will say:
The code server is ready.
Next, visit: http://127.0.0.1:9876/
Do so and follow instructions there.

Quick recap: Firefox nuked APIs that DevMode needed in 27.0, more than a year ago; Chrome announced approximately at the same time that they were deprecating NPAPI, that DevMode needs. They first made it impossible to update extensions containing NPAPI plugins in the Chrome Web Store a year ago, then they removed NPAPI support on Linux, and now they've just disabled NPAPI in Chrome 42, and will remove support entirely in 45, to be released in September.
"Classic" DevMode is deprecated for nearly 1½ year now; it really is time to move to SuperDevMode!

It's very likely that you are following word for word the instructions from Google at this link.
https://developers.google.com/eclipse/docs/getting_started
The instructions are out of date, and not valid for current versions of chrome.
Instead of selecting the option
Debug As > Web Application
use the option
Debug As > Web Application (GWT Super Dev Mode)
Google has "given up" on keeping their documentation up to date. I can only imagine how difficult it must be to ignore so many complaints.

From GWT Website ,http://www.gwtproject.org/release-notes.html#Release_Notes_2_6_1
GWT Development Mode will no longer be available for Chrome sometime in 2014, so we improved alternate ways of debugging. There are improvements to Super Dev Mode, asserts, console logging, and error messages.
This is because of newer chrome version.Use Super Dev Mode

NPAPI can be re-enabled in Chrome but it is going away.
Super dev mode supposed to be the way forward but I prefer the classic dev mode.
I'm using Windows. I installed a version of Chromium to stop this being a problem for me. The version I found to be the last of the 41 series was build 310958 which can be obtained here:
https://commondatastorage.googleapis.com/chromium-browser-snapshots/index.html?prefix=Win/310958/
You need to download the mini_installer.exe file. This version of Chromium will never update itself.
I recommend also that you make a copy of the GWT Developer Plugin that you can install without using Google's store because I believe it will be removed soon. I don't know how to do that, but one member of my team claims to have done it.

This issue is there, as Google has now completely discarded NPAPI which is required by GWT developer plugin which can been seen here:http://blog.chromium.org/2013/09/saying-goodbye-to-our-old-friend-npapi.html
Google now provides "Super Dev Mode" which does not require NPAPI.
For utilizing that you can check this blog: https://www.sencha.com/blog/getting-started-with-gwt-super-dev-mode/

Firefox Portable 26.0 supports the GWT Developer Plugin needed to work with Classic Dev Mode. You can install that along side an up-to-date browser.
For some reason, Super Dev Mode still does not work with some projects. This workaround has worked best for our team.

To get it working on Mac: download this version (Version 38.0.2084.0 (281482)) and install the plugin from the Chrome Web Store.

I did the same thing as suggested by davtom and it worked. I download the chrome-win32.zip file from https://commondatastorage.googleapis.com/chromium-browser-snapshots/index.html?prefix=Win/310958/ . I unzipped the files in a temp folder and run chrome.exe from there.
Thanks Davtom. This problem was driving me nuts for the past 3 hours. I rebooted, installed a different JDK, uninstalled and reinstalled the plugin but nothing worked.

To overcome this problem, Install very old version of chrome version 10 and run the app in super-dev mode. It would run as it should. There is no requirement to install GWT plugin for this version of chrome.

Browser Independent JavaScript debugger?

Does anyone know if a browser-independent debugger exists...
For example I have been using Firebug in Firefox but if the web developers use other browsers - nothing.
I was wondering if there is some kind of "global" debugger that can attach to any browser and just debug?
Any ideas?

The makers of Firebug also make Firebug Lite, a JavaScript file that you can include in your webpage. They say that it works in IE, Opera, and Safari.
Steve

Since each browser has its own engine with its own quirks and such, it's more or less impossible to have a universal debugger.
Aptana, which is really just a rebranded Eclipse for web projects, has a debugger built in to the IDE, and comes closes to that ideal--in theory. It uses the Eclipse platform's debugging interface, but it still needs to be plugged into a browser. In Firefox, this works by plugging into...Firebug's JS debugger. But, it allows you to set breakpoints and step through code directly from the IDE (quite helpful for external JS files). For IE debugging, you need to purchase the Pro version, so I can't vouch for its quality. AFAIK, it doesn't support other browsers.

Nearly all the browsers have there own JavaScript debugger. Simply because they don't provide the same API (if they provide some) for accessing the JavaScript engine.
So, to complete your list, you have DragonFly for Opera, and WebInspector for the browsers based on WebKit (like Safari).

For IE you can also use Visual Studio to debug JavaScript - and it's better than the Microsoft Script Debugger. I'm not sure if the express editions are also capable of this.
Added: Both VS2005 and VS2008 are capable of JS debugging, although the newer product is better of course.

I run a Webservice that allows developers to "debug" their Javascript in the sense of finding bugs via a Javascript validator. See the HTML5 IDE guide for details to set it up with the "global" vim editor.

You can debug javascript with express edition of visual studio. Look here for details

Short answer: I don't believe so, but I hope you find one because I'd love to have one.
However, if you haven't been following IE development lately, IE's JScript debugging is alot less painful now with IE8; it includes, by default, some dev tools that do pretty much everything that Firebug does with the exception of AJAX debugging (but I prefer a protocol analyzer anyways for AJAX). It even implements an executable console, which includes several of the console.X methods from Firebug!
The profiler, I'm tempted to say, is nicer than Firebug's - simply because it lets you export the results directly to CSV. The actual debugger is as full-featured as JS debuggers get, with stepping/breakpoints/direct input.
Also, I haven't heard Venkman mentioned yet; its the grand-daddy of Firefox debugging (and other Mozilla Javascript). Not as popular as Firebug, but it definitely has its followers.
Lots of good information on this page as well: wikibooks.org/wiki/JavaScript/Debugging

javascript depends on the browser you are using (thus compatibility problems). To debug explorer you may use this: Microsoft Script debugger

you can use jsdt, you can get some information from https://code.google.com/p/jsdt/

Dreamweaver JavaScript debugger

Does Dreamweaver CS 3 have a JavaScript debugger?
The only information on anything close is that it says I need to click on the
'preview/debug in browser' button which does open the page, but no debugging ever happens when the page has an error. I also see no way to set breakpoints or walk through the code.
MS Visual Web Developer (Visual Studio Express - which is free) has a debugger that you can attach to a process. So even if you are not developing in it, you can debug the JavaScript in any browser. It also has a very rich variable watch that allows you to drill down through all the decendants of an object for its respective values. I was hoping that Dreamweaver could at least match Visual Web Developer...
What is the experience using the Visual Studio debugger tools with non-Internet Explorer browsers?
Dreamweaver has no effective built-in debugger.
Firebug works great with non-Internet Explorer browsers
Visual Studio tools work great with ID browsers
What is the one that works well across the board?

Debuggers are specific to a particular interpreter/compiler, not to a language. The same language - in this case, JavaScript - can have more than one interpreter/compiler. In particular, each browser has their own.
So to debug JavaScript in Internet Explorer, you need an Internet Explorer debugger - either the one built into Internet Explorer, or one of the Visual Studio flavours. To debug JavaScript in Chrome, use Chrome's debugger. To debug JavaScript in Firefox, use Firebug. (And so on.)

There is nothing native to Dreamweaver that handles debugging JavaScript, but there are several other options out there for free.
The Firebug add-on for Firefox allows you to set breakpoints and step through JavaScript. Download and play with that, and you should find what you need. Here is a brief tutorial hitting on your points: Debug Javascript with Firebug

I solved most JavaScript problems using the Error Console in FireFox. I never got Dreamweaver's to work.

I agree with CheGueVerra, defenitively the best debugger is the "error console" in Firefox. If you want to make it even better, just download the Firefox Add-on Console². All you need to debug JavaScript code is there.

You can also use Firebug, which is in my opinion the best JavaScript debugger for Firefox even if there are still some issues sometimes (refer to my post a few days ago, Stack Overflow question Firebug debugger not working in Firefox 3.x?).

I assume you're looking for something where you can attach breakpoints and such... Well, without echoing the others (this can be done in Firebug), do try Aptana Studio. It can be run like a plugin on Eclipse and can be used to debug JavaScript.

Debug JavaScript in Eclipse

How can I debug JavaScript in Eclipse. I am using Eclipse 3.2.1. Everytime I click on the side it gives the option for adding bookmark but no break point.
Could anyone assist me on this?

In 2015, there are at least six choices for JavaScript debugging in Eclipse:
New since Eclipse 3.7: JavaScript Development Tools debugging support. The incubation part lists CrossFire support. That means, one can use Firefox + Firebug as page viewer without any Java code changes.
New since October 2012: VJET JavaScript IDE
Ajax Tools Framework
Aptana provides JavaScript debugging capabilities.
The commercial MyEclipse IDE also has JavaScript debugging support
From the same stable as MyEclipse, the Webclipse plug-in has the same JavaScript debugging technology.
Adding to the above, here are a couple of videos which focus on "debugging JavaScript using eclipse"
Debugging JavaScript using Eclipse and Chrome Tools
Debugging JavaScript using Eclipse and CrossFire (with FB)
Outdated
The Google Chrome Developer Tools for Java allow debugging using Chrome.

I don't believe Eclipse has a JavaScript debugger - those breakpoints are for Java code (I'm guessing you are editing a JSP file?)
Use Firebug to debug Javascript code, it's an excellent add-on that all web developers should have in their toolbox.

I'm not a 100% sure but I think Aptana let's you do that.

I tried to get aptana running on my ubuntu 10.4. Unfortunately I didn't succeed. Chrome on the other hand, has an eclipse plugin that lets you debug javascript that's running in a chrome instance. Works very well.
YOu'll have to install the eclipse plugin you'll find here:
http://code.google.com/p/chromedevtools/
Set Breakpoints in the javascript sources you edit in eclipse and browser your page in chrome. As soon as a javascript breakpoint is hit, the eclipse debugger halts and lets you step into, step over, browse the variables etc. Very nice!

JavaScript is executed in the browser, which is pretty far removed from Eclipse. Eclipse would have to somehow hook into the browser's JavaScript engine to debug it. Therefore there's no built-in debugging of JavaScript via Eclipse, since JS isn't really its main focus anyways.
However, there are plug-ins which you can install to do JavaScript debugging. I believe the main one is the AJAX Toolkit Framework (ATF). It embeds a Mozilla browser in Eclipse in order to do its debugging, so it won't be able to handle cross-browser complications that typically arise when writing JavaScript, but it will certainly help.

Use the debugging tools supported by the browser. As mentioned above
Firebug for Firefox
Chrome Developer Tools from Chrome
IE Developer for IE.
That way you can detect cross-browser issues. To help reduce the cross-browser issues, use a javascript framework ie jQuery, YUI, moo tools, etc.
Below is a screenshot (javascript-debug.png) of what it looks lime in Firebug.
1) hit 'F12'
2) click the 'Script' tab and 'enable it' (if you are already on your page - hit 'F5' to re-load)
3) next to the 'All' drop down, there will be another dropdown to the right. Select your javascript file from that dropdown.
In the screenshot, I've set a break-point at line 42 by 'left-mouse-click'. This will enable you to break, inspect, watch, etc.

It's possible to debug JavaScript by setting breakpoints in Eclipse using the AJAX Tools Framework.

MyEclipse (eclipse based, subscription required) and Webclipse (an eclipse plug-in, currently free), from my company, Genuitec, have newly engineered (as of 2015) JavaScript debugging built in:
You can debug both generic web applications and Node.js files.

For Node.js there is Nodeclipse 0.2 with some bug fixes for chromedevtools