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 work in an application that is JavaScript intense. So to debug , I end up using many alerts. Are there other better ways to debug ? What methods do you use ?
For JavaScript, debugging is a sinch in most browsers:
IE - Where you really need to debug, F12 is the console. You can call console.log, console.debug, console.error and a few others, and it will print out good data. When you call console.log on an object, good consoles will print out the property break-down of the object. There's also an active dom inspector so that you can see what's going on as the script is running.
Firefox - Get the Firebug addon. It is my favorite console of them all. Does everything I've ever dreamed of needing, and a few more features.
Chrome - Built-in console, inspect element on the page to see the breakdown of the DOM live.
Opera - Built-in console
Safari - Add Firebug Lite to the page, and you'll have a JS driven version of Firebug. Safari has built-in dev tools similar to Chrome, however they need to be enabled.
For all browsers, you can add Firebug Lite, but I really only use it for IE and Safari.
Most modern browsers include a console, which can help with displaying syntax errors and the like. Of course, there's Firebug.
Use Firefox as your main development platform and open the Web Console. Error messages are printed there.
Chrome also has a console, but it doesn't have Firebug, a Firefox add-on used for web development. While the Web Console that comes with Firefox will display errors in Javascript, Firebug will also help with inspecting the HTML for your page, and there are even extensions to Firebug like Flashbug for working with Flash components on your page.
Obviously you'll want to check your web apps in all browsers eventually, but do most of your developing in Firefox.
Hi I am beginner in JS, Using linux ( Firefox, Chrome), Is there any way by which I get what are the syntax errors are there in my JS, the way I get my C syntax error in GCC?
Install Firebug, it will save your life!!!
Anyway in Firefox you can look into the error console in the tools menu and in Chrome you can open the javascript console in the tools menu.
if you use a script-editor like netbeans, it will show syntak errors on typing. for testing in a browser, i would suggest using firebug (and i'm sure theres something similar for chrome, too).
Chrome : ctrl + shift + j : this will bring up the JavaScript console. You can click the scripts tab to review page scripts.
FireFox : Install firebug and run it to get a similar console to the above stated chrome utilities.
IDE : You may wish to use netbeans or Eclipse which both offer syntax highlighting and code completion for JavaScript. These will indicate syntax errors at code time.
jslint/jshint : These tools offer code suggestions to improve your code quality and reduce the possibility of errors (logic) but will also break on syntax and other errors.
I don't get any error in firefox or firebug, but yet in IE I get invalid argument for some reason, and I can't figure out what is the invalid argument, the javascript halts when "error" is discovered .. what can I do to debug it ?
Firebug Lite is awesome.
If you're running IE8, you can use the developer tools by pressing F12. You can also download them for IE7.
If you have Visual Studio you can use it directly to debug javascript.
First uncheck the advanced IE settings "Disable script debugging" then attach VS to the IExplorer process (Tools => Attach To Process then find the process).
Clearly this is not the easiest option here but can work in a pinch. If you are already using VS to do your development just disabling script debugging and launching your website with IE will allow the JS debugger to work automatically.
Enable the script debugger in IE.
Note that with jquery or mootools 90% of the IE bugs (when it works fine in FF) is because I didn't explicitly extend an element I was working with.
In firefox you can just say element.jquery_function(), however with IE the element must be explicitly extended with element = $(element), or just get in the habit of writing $(element).jquery_function().
You didn't give enough info to debug your problem, but i'd guess that is a good place to start.
note: I use mootools but i'm assuming this also applies to jquery
get internet explorer 8, or the internet explorer developer toolbar
I prefer DebugBar over Firebug Lite.
I would use a cross-browser logging library such as my own log4javascript.
If you're on IE 8, that comes bundled with developer tools.
If you're on IE 7 or 6, there's the free IE Developer Toolbar from Microsoft.
There's also Companion.JS for IE 6/7.
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.