Is there any Javascript Debugging API?

Is there any Javascript Debugging API? - javascript

I want to write some Debugging Tools. First of them would be a Smalltalk like object browser. But i need some API's to extract methods from objects, dumping local and global variables and the stacktrace.
It's okay if this is Webbrowser specific. But please tell me which Webbrowser has an documented API for this.
EDIT: I would prefer a pure Javascript API, but something like an COM interface is also fine as long as it is well documented and has sample code.

Take a look at https://addons.mozilla.org/en-US/firefox/addon/216/ (Javascript Debugger) or FireBug for Firefox. Both are open source and can give you the details.
Another option is the Web Inspector for Chrome and Safari, Dragonfly for Opera, or F12 Dev Tools for Internet Explorer.
All of them can give you ideas on how to implement your own debugging tools.

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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.

Is it possible to have an in-browser debugger?

Is it presently possible to programatically debug Javascript from within a browser window? I wish to be able to breakpoint/inspect Javascript from an in-browser window panel.
EDIT:
I should have been more specific: I'm loogking for an api to be used from within the page context, i.e. JS code in browser can add it's own breakpoints.

It's already built-in in Chrome and Safari and, to an extent, in Firefox. There exists multiple plugins for this. Look at Firebug for Firefox, and F12 Developer Tools for IE.

Utilize your browser's developer tools:
Use browser-specific developer tools, which allow you to identify runt-time errors, set up breakpoints, and run performance diagnostics. To make debugging much easier, don’t minify the JavaScript during development deployment.
In IE: IE Developer Tools
Or install the Google Chrome Frame which allows IE to process JavaScript with Google Chrome’s V8 engine and allows the user to debug with the Chrome Developer Tools
In FF: Firebug extension
In Chrome: Chrome Developer Tools
Use a static code checker, like JSLint, to verify that JavaScript code complies with generally accepted coding rules.
Use JsFiddle to easily test and improve JavaScript/jQuery functions (http://jsfiddle.net/)
This has a JSLint option for parsing the JavaScript

If you're using Firefox, try FireBug.
If you're using Chrome, it has an in-built debugger.
For IE there's something called DebugBar and F12 Developer Tools.

just go for firebug.
http://getfirebug.com
you can also have breakpoints
http://getfirebug.com/doc/breakpoints/demo.html

In Google Chrome -> Right Click -> Inspect Element
or
Firebug for Firefox

F12 in almost any modern browser.

Yes there are possiblities to do that.
For Chrome for example you can use the
the chrome.debugger extension to attach your own debugger-implementation to a website, that runs in another tab/window. Once you are attached, you control the debugger vie a JSON based remote debugging protocol. The biggest problem I would say is, that you have to pipe all your communication through an HTML-Element. This is the case because the extension can only communicate with a content script and the content script can only access DOM-specified attributes.
John Bartons chrome extension crx2app tries to simplify the access to the extension API. I don't know how much it actually supports, but it might also be helpful for developing a Debugger that runs from within the browser itself.
As far as I know Firefox also provides access to its internal JavaScript-Engine Spidermonkey and by extension to its debugging mechanism. But that might in fact collide with a FireBug plug-in, because you can usually only attach one listener to the Debugger. But with the recent release of the new Firefox built-in JavaScript-Debugger, which provides remote-access, this might be no longer a problem.

JavaScript Debugging

I am using Visual Studio 2010 Express Developer edition. Developing a Website. I'm using JavaScript in my .aspx pages. How to put breakpoints in Javascript.
Any help is highly appreciated.
Thank You

If you are using Web Developer Express you can do this within Visual Studio - here are the instructions.
If you are using a different Express edition, you will have to use a browser tool for client side debugging - for example Firebug. IE and Chrome have built in developer tools that have this kind of functionality.

You can use your browser's debugger :
For Firefox, you can use Firebug (http://getfirebug.com).
Chrome contains an embedded debugger.

From memory you will need to do the following to get JS debugging in Visual Studio:
Ensure that Internet Explorer has script debugging enabled (somewhere in tools -> options) - this is the bit most people don't know about.
Then go into the .aspx and add a breakpoint by clicking in the left margin of the line of Javascript you want to debug.
Launch the website project in debug mode (F5) and hit the page using IE - your breakpoint should then hit.
As noted by other people you can use a host of other (usually better) tools that come with other browsers.

If you're debugging IE, use the following links:
http://aspnetlive.blogspot.com/2010/02/debugging-java-script-in-visual-studio.html
http://msdn.microsoft.com/en-us/library/7seh8d72.aspx
For Firefox, I recommend Firebug.
http://getfirebug.com/
For Chrome, you can use the Developer's Tools debugger.
Update
Here's a comprehensive list of debugging JavaScript in each browser:
http://siliconforks.com/doc/debugging-javascript/

I like Firebug, plugin for Firefox.

I think the idea is to use the Attach to process option of the Debug menu; but to be honest, I've never had much luck getting VS to do this fluidly (though I remember it working at times, success is spasmodic at best), maybe tabbed browsing has made this an issue - I'm unsure.
One thing I can recommend, however, and we might get a few hecklers, are the Developer Tools of IE9. If nothing else, IE9 has become an invaluable tool for me recently simply because of its Javascript debugging capabilities. Here's an MSDN article to get you started with this.

Using FireBug & Internet Explorer Debuging tool let you debug and put breakpoint on your javascript files

Is there any way to check the current variable status and other debugging methods?

Is there any way to check the current variables value and such using some sort of debugger? I have an error in my code, and I want to see if it is a variable causing it, and if not, I would like to figure out what is. What are some really good debuggers for JS anyone knows of?

You can take a look at Firebug http://getfirebug.com/

I agree with gpmattoo Firebug is great. Internet Explorer has a javascript debugger built in (hit F12) Chrome has one built in too but there is also a Firebug lite that should work if you like Firebug for Firefox

Visual Studio (and Visual Web Developer) has support for JS debugging.
More lightweight, however, you can probably find something that'll plug in to your browser. IE has a JS debugger built-in (as of IE8 I think), Firebug is probably the way to go in Firefox, and I'm pretty sure Chrome also has a built-in debugger.

You can use Firebug under Firefox or the chrome inspector (right click on the page and inspect...) under Chrome.
You will then need to use some logging if the variable is a local one (console.log())

How can I debug my JavaScript code? [closed]

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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.

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.

Develop Reference

JavaScript is the programming language of the Web.