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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.
Closed. This question needs to be more focused. It is not currently accepting answers.
Want to improve this question? Update the question so it focuses on one problem only by editing this post.
Closed 6 years ago.
Improve this question
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 have an ASP.NET Web Application created with Visual Studio 2013. I am attempting to debug JavaScript in a CSHTML file. However, whenever I launch the webpage, any breakpoint turns into a red circle arrow and states, "The breakpoint will not currently be hit. No executable code of the debugger's target code type is associated with this line. Possible causes include: conditional compiliation, compilier optimizations, or the target architecture of this line is not supported by the current debugger code type."
Recently, the project was switched over to support MVC and RAZR, neither of which I know well, and this is exactly when this issue began. However, searching those have yielded results that don't fix my issue.
Web.config:
<compilation debug="true"...>
I know I can debug JavaScript with Firebug or some other browser tool, but I would much rather stick with Visual Studio's debug as that is what I am used to.
So, apparently this is a "known issue" that will be fixed as soon as possible. A temporary work around that works for "some" people is making sure any Javascript is in a separate file.
It is caused by having RAZR and Javascript in the same file and Visual Studio 2013 not being able to handle debugging in that instance.
I don't know what your particular problem is, but if you want to force a debug breakpoint to always happen, add debugger; to the line that you want it to stop on, and it will stop. This is regardless of where the JS is located (in a .js file, .html, cshtml, etc.)
Here is a blog post about it:
http://sumitmaitra.wordpress.com/2013/12/11/quickbytes-visual-studio-2013-and-javascript-debugging/
I also agree that JS should go in a .js file (which I've never had a problem adding a break point in a .js file), but for quick prototyping, this is a solution you can use.
If that still doesn't work, you can always you the F12 tools
The only browser that allows debugging a javascript file from Visual Studio is Internet Explorer. (this is what I found out after testing my application on different browsers)
I put my javascript in a separate file and debug with IE otherwise it will not work.
For some reason chrome doesnt allow you to step into the javascript.
One additional thing to check. If you have a App_Start|BundleConfig.cs (which came with MVC 4 - or maybe 3), set BundleTable.EnableOptimizations to false (or, like I did, wrap it in an #if !DEBUG #endif and take the default setting).
I tried and failed to use Chrome and then IE and ended up using the Firebug addon in Firefox, and I was able to debug and set breakpoints in my JS with no problems (in an MVC6 app on Visual Studio 2015 where this is apparently still an issue?!)...
FYI - When I tried to debug my JS in Chrome using the F12 Developer Tools, it told me it was not an option as the Debugger was already attached to another process...
For people coming here in 2017, I want to share that I had this same issue with VS2017 Enterprise RC, and with VS 2015 Community with any browser but Internet Explorer. Using IE did the trick for me.
Also, in VS2017, I had to add a debugger statement to get VS start looking at debug points.
Finally, I'd like to ask at least a comment from people voting down.
After having written large amounts of code in Intellij Idea Ultimate edition, I often want to test a method, or big pieces.
I often resort to having to paste the code in firebug in firefox, a small annoying cramping space, with no editor features. If the code needs adjusting I need to do it there, test again, copy and insert into Intellij Idea.
Is it possible to run firebug like console code, right in Intellij ? Similar to in Java debug mode with the Inspect tool ? It would have been really useful, even more useful just to highlight some code and press run.
Browser support is not important, any browser will do.
Is this possible already? Is there an Intellij plugin for this? Why not? :(
Thanks!
If the browser is not important, you could use the node.js plugin as a javascript repl / debugger. Keep in mind that it will not provide an HTML DOM so if your code makes jQuery calls, it will not run out of the box. Otherwise, if it's just plain javascript, it will run just fine.
Just stumbled over this. A bit late but anyway:
Debugging Javascript, editing values, etc. works fine using intelliJ with its Chrome Plugin.
See https://www.jetbrains.com/idea/webhelp/configuring-javascript-debugger.html or
http://blog.jetbrains.com/idea/2011/03/intellij-idea-debugging-javascript-in-google-chrome/
You can set breakpoints to stop where you want and can from there evaluate expressions and all the other stuff you would like to do.
Hope this is what you wanted to know.
I want to develop a Thunderbird 12 addon. I have written a simple JS function. (File: "chrome\content\myApp.js"). This file is called by a xul-'onclick'-event. I have packed my addon as a .xpi file and then I have installed it. (All ok, it's working!)
Now I'd like to debug my JS functions. But I can't find a working JS Debugger.
I have tried this addons:
"Javascript Debugger"(0.9.89). Not working: The thread doesn't stop at my breakpoints.
Firebug(1.7.2) + Chromebug(1.7.2). Not working: I can't enable the JS debug tab. (Update: I also can't set/change a "context")
Happily we once again have a debugger for Thunderbird! You need a newer/nightly version, but hanks to Philipp Kewisch's summer of code project we can use the Firefox debugger to remotely debug Thunderbird. More details at the following links:
http://kewisch.wordpress.com/2013/09/22/thunderbird-developer-tools-wrapup/
http://kewisch.wordpress.com/2013/06/13/the-thunderbird-remote-debugger-is-alive/
https://bugzilla.mozilla.org/show_bug.cgi?id=876636
If you are really committed to TB12, you can debug on the nightly then test on TB12.
Unfortunately there's no such thing as a decent debugger for chrome (addons) javascript. Chromebug has been long abandoned, venkman doesn't work, addon authors are a little bit stranded there. A new debugging API called JSD2 is in the works at Mozilla, but it looks from the wiki that it's mostly targeted at content (webpage) javascript, not chrome (addons) javascript, at least not in the short term.
Fortunately, a very recent effort seems to have paid off: there's now a way to execute easily small snippets of javascript in a decent console-like window. See http://mikeconley.ca/blog/2012/05/11/scratchpad-ported-to-thunderbird-kinda/ for the details :).