How can I debug my JavaScript code

Question

When I find that I have a problematic code snippet  how should I go about debugging it

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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

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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

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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

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I use a few tools  Fiddler  Firebug  and Visual Studio  I hear Internet nbsp Explorer nbsp 8 has a good built-in debugger

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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

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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

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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

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Internet nbsp Explorer nbsp 8  Developer Tools - F12    Anything else is second rate in Internet nbsp 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

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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

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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 nbsp 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

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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

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I found the new version of Internet nbsp Explorer nbsp 8  press F12  is very good to debug JavaScript code   Of course  Firebug is good if you use Firefox

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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

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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

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I used to use Firebug  until Internet nbsp Explorer nbsp 8 came out  I m not a huge fan of Internet nbsp 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

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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

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Firebug is one of the most popular tools for this purpose

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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

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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

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I m using Venkman  a JavaScript debugger for XUL applications

[javascript] How can I debug my JavaScript code?

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