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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.
I'm developing in Javascript for quite a long time now. Usually when I hit an error in IE I know roughly where it originated even if the message received from IE is useless bunch of text. When I don't know where the error originated, I usually try to "delete" parts of my code, until the error doesn't repeat itself, and that start manually checking line by line until I find the error.
I'm sure that it's far from the best approach, so I'd like to ask you how you debug error like these:
If you are using IE8+, you can press F12 on a page to open the Developers Tools.
This contains a JavaScript debugger, much like Firebug & Chrome Dev Tools
EDIT:
In response to the comment under the question, if IE is throwing a cryptic error that you are unsure of, there is a couple of steps I would do.
Is it an IE only error? Does the same error occur in Firefox? Chrome?
Is the error occurring in a 3rd party library. If you believe it is, use an un-minified version of the library.
Can you replicate the error outside of your website? Can you make the error occur in a http://jsfiddle.net/ for instance?
If you still can't narrow down the issue, post a question on SO with your code, any error messages, and expectations of the result.
HTH
Try using non-minified version of jQuery - it will give you a better idea where exactly the error is. Also, if you use VS 2010 to debug your js code in IE, it will break at the error line. This always works fine for me.
IE is the only browser that I've managed to successfully use the fantastic Visual Studio script debugger with - in my experience Visual Studio is hands down the best script debugger out there, so quite often I find myself in the reverse situation to you (running broken scripts in IE just so I can use the script debugger)
See How to debug JavaScript in Internet Explorer for instructions on how to use Visual Studio Express to debug scripts in IE - if you own a full edition of Visual Studio then its much simpler (just attach to IE as normal).
All I get is this:
An error has occurred in the script on this page
line: 31
char: 5
error: expected identifier, string or number
code: 0
url: http://example.com
It's difficult to find that line since I have several external and internal scripts.
How can I debug this? (By the way, the scripts have no errors in chrome or firefox or ie8+)
You can download IETester which lets you use various versions of IE (6-8) and has built-in developer tools. They're not spectacular, but they're better than the nothing that old versions of IE provide.
Try debugging it with IE8's Developer Tools (press F12). You should be able to step through the JS code (from all your scripts) to find out what is wrong with line 31 / char 5.
If it says it's on the page, then it means the page itself. If it was in an external file, it would indicate that the error occured in something.js.
Open up your webpage, right click on the page and select "View Source". Go to line 31 and try and see what's wrong with it.
You can also enable debugging for IE at
(Tools -> Options -> (uncheck) Disable script debugging (Internet Explorer).
After you disable it, IE will prompt you to debug when error occurs. Then you can debug with VS or MS script Editor.
Use Firefox with Firebug. Look at every external script around the line number mentioned in the error dialog. Trailing commas every time!
I got the same issue when try to to figure out what 's wrong with the code on IE7, after spending hours research and trying many different solutions, finally, the following two bars are great tools for me:
1) http://www.debugbar.com/
provide a convenience UI to with feature like source, style, DOM, Script, HTML check.
2) http://www.my-debugbar.com/wiki/CompanionJS/Installing
The main advantage of this is providing a console for IE6 or IE7, so you can see what are the error (in the console log)
Note:
they are both free
screen shot of both two toolbars
I need to debug JavaScript in Internet Explorer 7.
Unfortunately, its default debugger doesn't provide me with much information. It tells me the page that the error showed up on (not the specific script) and gives me a line number. I don't know if that is related to my problem.
It'd be nice if it could narrow down the error to a line number on a specific script (like Firebug can).
Is there an addon to debug JavaScript in IE7 like Firebug does in Firefox?
Thank you!
See also:
Does IE7 have a “developer mode” or plugin like Firefox/Chrome/Safari?
Web Development Helper is very good.
The IE Dev Toolbar is often helpful, but unfortunately doesn't do script debugging
The hard truth is: the only good debugger for IE is Visual Studio.
If you don't have money for the real deal, download free Visual Web Developer 2008 Express EditionVisual Web Developer 2010 Express Edition. While the former allows you to attach debugger to already running IE, the latter doesn't (at least previous versions I used didn't allow that). If this is still the case, the trick is to create a simple project with one empty web page, "run" it (it starts the browser), now navigate to whatever page you want to debug, and start debugging.
Microsoft gives away full Visual Studio on different events, usually with license restrictions, but they allow tinkering at home. Check their schedule and the list of freebies.
Another hint: try to debug your web application with other browsers first. I had a great success with Opera. Somehow Opera's emulation of IE and its bugs was pretty close, but the debugger is much better.
you might want to try
microsoft script debugger
it's pretty old but it's quite useful in the sense if you stumble on any javascript error, the debugger will popup to show you which line is messing up. it could get irrating sometimes when you do normal surfing, but you can turn if off.
here's a good startup on how to use this tool too.
HOW-TO: Debug JavaScript in Internet Explorer
I've found DebugBar.
Not as good as Firebug, but close.
In IE7, you can bring up firebug lite for the current page by pasting the following in the address bar:
javascript:var firebug=document.createElement('script');firebug.setAttribute('src','http://getfirebug.com/releases/lite/1.2/firebug-lite-compressed.js');document.body.appendChild(firebug);(function(){if(window.firebug.version){firebug.init();}else{setTimeout(arguments.callee);}})();void(firebug);
See http://getfirebug.com/lite.html.
Microsoft Script Editor is indeed an option, and of the ones I've tried one of the more stable ones -- the debugger in IE8 is great but for some reason whenever I start the Developer Tools it takes IE8 a while, sometimes up to a minute, to inspect my page's DOM tree. And afterwards it seems to want to do it on every page refresh which is a torture.
You can inspect contents of variables in Microsoft Script editor: if you poke around under Debug > Window you can turn on local variable inspection, watching etc.
The other option, Visual Web Dev, while bulky, works reasonably well. To set it up, do this (stolen from here):
Debugging should be turned on in IE. Go into Tools > Internet Options > Advanced and check that Disable Script Debugging (Internet Explorer) is unchecked and Display a notification about every script error is checked
Create a new empty web project inside of VWD
Right-click on the site in the Solutions Explorer on the top right, go to Browse With and make sure your default browser is set to IE (it's reasonable to assume if you're a web developer IE is not your default browser in which case that won't be the default.. by default)
Hit F5, IE will open up. Browse to the page you want to debug.
VWD will now open up any time you have a script error or if you set a breakpoint in one of the JS files. Debug away!
UPDATE: By the way, if you experience the same slowdowns as me with IE8's otherwise decent debugger, there is a workaround -- if you encounter or make IE encounter an error so that it pops up the "Do you want to debug" dialogue and hit Yes, the debugger will come up pretty much instantly. It seems like if you go "straight" into debugging mode the Dev Tools never inspect the DOM. It's only when you hit F12 that it does.
IE8 has much improved developer tools. Until then it's best to write javascript for firefox first and then debug IE using alert() statements.
Microsoft Script Editor can be used to debug Javascript in IE. It's less buggy than Microsoft Script Debugger but has the same basic functionality, which unfortunately is pretty much limited to stepping through execution. I can't seem to inspect variables or any handy stuff like that. Also, it only shipped with Office XP/2003 for some bizarre reason. More info here if you're game.
I downloaded the Visual Web Developer 2008 Express Edition mentioned by Eugene Lazutkin but haven't had a chance to try it yet. I'd recommend trying that before Script Editor/Debugger.
It's not a full debugger, but my DP_DEBUG extensions provides some (I think) usful functionality and they work in IE, Firefox and Opera (9+).
You can "dump" visual representations of complex JavaScript objects (even system objects), do simplified logging and timing. The component provides simple methods to enable or disable it so that you can leave the debugger in place for production work if you like.
DP_Debug
The IE9 developer tools worked for me. Just set the "Browser Mode" menu item to IE7.
Hey I came across the same problem and found this the application IETESTER. It's pretty awesome, it's an app that has IE 5.5,6, and 7 bundled into it. It doesn't matter what IE version you currently have. This allows you to have multiple versions side by side.
If you enable javascript debugging in IE options and have Visual Studio installed you can even debug the javascript in VS with all the debug options available to you(watches, conditional breakpoints ,etc.)
If you want to start debugging before an error occurs you simply have to put the line
debugger;
into your JS code and this bring you into VS to begin debugging after this statement.
This is absolutely amazing to me for testing backward compatibility for JS code.
Use Internet Explorer 8. Then Try the developer tool.. You can debug based on IE 7 also in compatibility mode
FireBug Lite:
http://getfirebug.com/firebuglite
The answer is simple.
Get Internet Explorer 9
Press F12 to load up Developer Tools
Switch the browser mode to IE7
Running your code through a Javascript static analysis tool like JSLint can catch some common IE7 errors, such as trailing commas in object definitions.
IE8 Developer Tools are able to switch to IE7 mode
If you still need to Debug IE 7, the emulation mode of IE 11 is working pretty well.
Go to menu: Dev Tools, then to emulation and set it.
It also gives error line information.
The following tools works great for me:
1) http://www.debugbar.com/
Provide a convenience UI to with feature like source, style, DOM, Script, HTML check. It also show the actual error in your JS file (which line, which file).
2) http://www.my-debugbar.com/wiki/CompanionJS/Installing
Provide a console for IE6 or IE7 ( which originally does not support)
Screenshot
I have a few scripts on a site I recently started maintaining. I get those Object Not Found errors in IE6 (which Firefox fails to report in its Error Console?). What's the best way to debug these- any good cross-browser-compatible IDEs, or javascript debugging libraries of some sort?
There's no cross-browser JS debugger that I know of (because most browsers use different JS engines).
For firefox, I'd definitely recommend firebug (http://www.getfirebug.com)
For IE, the best I've found is Microsoft Script Debugger (http://www.microsoft.com/downloads/details.aspx?familyid=2f465be0-94fd-4569-b3c4-dffdf19ccd99&displaylang=en). If you have Office installed, you may also have Microsoft Script Editor installed. To use either of these, you need to turn on script debugging in IE. (uncheck Tools -> Internet Options -> Advanced -> Disable Script debugging).
You could also use Firebug Lite - which will work in IE & Opera. It's an external lib that will help you track down problems. It's sometimes more convenient than dealing with the MS Script Debugger.
Firebug
It's only for firefox but it should let you figure out what's happening on IE especially once you have the script line numbers.
You can use Visual Studio and enable debugging in browser
You can install FireBug plugin for Firefox, it's really good!
You can try to install IE8 beta 2 and use it in compatibility mode with built-in debugger.
Also in any line of your JS code you can write
debugger;
and this will be threated as breakpoint for any of the debug tools you use.
Cheers!
Aptana Studio provides JavaScript debugging for Firefox and IE
Firebug is the best all around client-side debugger. I frequently use it to debug CSS code as well as javascript. It allows you to easily find offending areas of code. I especially like the ability to modify tag attributes in the firebug pane and see the effects immediately before committing. Very useful for anyone designing websites.
You could use this tool apparently - Microsoft Script Debugger
Personally I try to go through the code and figure out what's going on - it gives you the line number where it goes wrong right?
To make the Microsoft Script Debugger more user friendly (and to add javascript error messages that actually are helpful to IE), I highly recommend Companion.JS.
Firebug seems to be the most useful so far. When a page is running on firebug, it can be very handy to log messages into firebug via javascript calls to console.log('your log message'); but don't execute that code in IE since the console object is only in scope when firebug is running.
For IE, other folks have mentioned the Script Debugger. Although it is not primarily for javascript debugging, it can be useful to also add the IE developer toolbar, which allows you to easily and dynamically inspect the style and other properties of your page's DOM.
In response to mopoke, for IE6 you definitely want to use Visual Studio for debugging if you can get it. For all intents and purposes, the MS script debugger is useless. You're better off using some form of tracing (not alerts) than using the MS script debugger. Dojo Toolkit, for instance, provides a debug console for tracing, but you can write your own by dumping messages to a secondary window or div.
The script debugger needlessly prompts you on each error in IE6 and even then doesn't give you enough state context to make it useful in a sufficiently complex JS app. Visual Studio is more tightly integrated and much friendlier. Just my experience.