How do I debug Node js applications

Question

How do I debug a Node js server application   Right now I m mostly using alert debugging with print statements like this   sys puts sys inspect someVariable      There must be a better way to debug  I know that Google Chrome has a command-line debugger  Is this debugger available for Node js as well

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There is the new open-source Nodeclipse project (as a Eclipse plugin or Enide Studio):

Nodeclipse became #1 in Eclipse Top 10 NEW Plugins for 2013. It uses a modified V8 debugger (from Google Chrome Developer Tools for Java).

Nodeclipse is free open-source software released at the start of every month.

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The V8 debugger released as part of the Google Chrome Developer Tools can be used to debug Node js scripts  A detailed explanation of how this works can be found in the Node js GitHub wiki

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Node js version 0 3 4  has built-in debugging support   node debug script js  Manual  http   nodejs org api debugger html

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Use this commands  DEBUG LEVEL all node file js DEBUG   node file js node file js --inspect

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Just for completeness   The PyCharm 3 0   Node js Plugin offers an awesome development   run   debug experience

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Chrome dev tools it   s an easy way to start debugging your client-side code  Here are the steps on what you need to Debug node app using chrome dev tools  check for more details here

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I created a neat little tool called pry js that can help you out   Put a simple statement somewhere in your code  run your script normally and node will halt the current thread giving you access to all your variables and functions  View edit delete them at will   var pry   require  pryjs    class FizzBuzz    run  - gt      for i in  1  100        output            eval pry it     magic       output     Fizz  if i   3 is 0       output     Buzz  if i   5 is 0       console log output    i    bar  - gt      10  fizz   new FizzBuzz   fizz run

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IntelliJ works wonderfully for Node js   In addition  IntelliJ supports  Code Assistance  well

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node-debug -p 8888 scriptFileName js

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Assuming you have node-inspector installed on your computer  if not  just type  npm install -g node-inspector   you just have to run   node-inspector  amp  node --debug-brk scriptFileName js   And paste the URI from the command line into a WebKit  Chrome   Safari  browser

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node-inspector could save the day  Use it from any browser supporting WebSocket  Breakpoints  profiler  livecoding  etc    It is really awesome   Install it with   npm install -g node-inspector   Then run   node-debug app js

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ndb is an improved debugging experience for Node js  enabled by Chrome DevTools  https   github com GoogleChromeLabs ndb

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Visual Studio Code will be my choice for debugging  No overhead of installing any tools or npm install stuff  Just set the starting point of your app in package json and VSCode will automatically create a configuration file inside your solution  It s build on Electron  on which editors like Atom are built      VS Code gives similar debugging experience as you might have   had in other IDEs like VS  Eclipse  etc

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A lot of great answers here  but I d like to add my view  based on how my approach evolved   Debug Logs  Let s face it  we all love a good console log  Uh oh  if you reached here  you better run    and sometimes that works great  so if you re reticent to move too far away from it at least add some bling to your logs with Visionmedia s debug    Interactive Debugging  As handy as console logging can be  to debug professionally you need to roll up your sleeves and get stuck in  Set breakpoints  step through your code  inspect scopes and variables to see what s causing that weird behaviour  As others have mentioned  node-inspector really is the bees-knees  It does everything you can do with the built-in debugger  but using that familiar Chrome DevTools interface  If  like me  you use Webstorm  then here is a handy guide to debugging from there   Stack Traces  By default  we can t trace a series of operations across different cycles of the event loop  ticks   To get around this have a look at longjohn  but not in production     Memory Leaks  With Node js we can have a server process expected to stay up for considerable time  What do you do if you think it has sprung some nasty leaks  Use heapdump and Chrome DevTools to compare some snapshots and see what s changing     For some useful articles  check out   RisingStack - Debugging Node js Applications Excellent article by David Mark Clements of nearForm   If you feel like watching a video s  then   Netflix JS Talks - Debugging Node js in Production Interesting video from the tracing working group on tracing and debugging node js Really informative 15-minute video on node-inspector   Whatever path you choose  just be sure you understand how you are debugging       It is a painful thing   To look at your own trouble and know   That you yourself and no one else has made it        Sophocles  Ajax

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Visual Studio Code has really nice Node js debugging support  It is free  open source and cross-platform and runs on Linux  OS nbsp X and Windows   You can even debug grunt and gulp tasks  should you need to

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I personally use JetBrains WebStorm as it s the only JavaScript IDE that I ve found which is great for both frontend and backend JavaScript   It works on multiple OS s and has Node js debugging built-in  as well as a ton of other stuff  http   www jetbrains com webstorm features index html    My only  issues  wishlist items are were    It seems to be more resource hungry on Mac than Windows It no longer seems an issue in version 6  It would be nice if it had Snippet support  like those of Sublime Text 2 - i e  type  fun  and tap  tab  to put in a function  See  WickyNilliams comment below - With Live Templates you also have snippet support

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I put together a short Node js debugging primer on using the node-inspector for those who aren t sure where to get started

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There is built-in command line debugger client within Node js  Cloud 9 IDE have also pretty nice  visual  debugger

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My original response was couple of years ago pre visual studio   So  Using  GOOD  by hapi is  a great logging package but for debugging use visual studio   original response  some long time ago   I would use GOOD by Walmart Labs  It will do the job  and it s very flexible   var hapi   require  hapi    var good   require  good    var server   hapi createServer  localhost   5000      server route  SOME ROUTE HERE    server start     var options     subscribers         console                   ops    request    log    error         http   localhost logs     log            server pack require  good   options  function  err         if   err            console log  Plugin loaded successfully

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Node js Tools for Visual Studio 2012 or 2013 includes a debugger   The overview here states  Node js Tools for Visual Studio includes complete support for debugging node apps    Being new to Node js  but having a background in  NET  I ve found this add in to be a great way to debug Node js applications

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I wrote a different approach to debug Node js code which is stable and is extremely simple  It is available at https   github com s-a iron-node     An opensource cross-platform visual debugger   Installation   npm install iron-node -g   Debug   iron-node yourscript js

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You may use pure Node js and debug the application in the console if you wish   For example let s create a dummy debug js file that we want to debug and put breakpoints in it  debugger statement     x000D   x000D  let a   5  x000D  debugger  x000D   x000D  a    2  x000D  debugger  x000D   x000D  let b   10  x000D  debugger  x000D   x000D  let c   a   b  x000D  debugger  x000D   x000D  console log c   x000D   x000D   x000D    Then you may run this file for debugging using inspect command   node inspect debug js   This will launch the debugger in the console and you ll se the output that is similar to    lt  Debugger listening on ws   127 0 0 1 9229 6da25f21-63a0-480d-b128-83a792b516fc  lt  For help  see  https   nodejs org en docs inspector  lt  Debugger attached  Break on start in debug js 1  gt  1  function  exports  require  module    filename    dirname    let a   5    2 debugger    3   You may notice here that file execution has been stopped at first line  From this moment you may go through the file step by step using following commands  hot-keys     cont to continue  next to go to the next breakpoint  in to step in  out to step out pause to pause it   Let s type cont several times and see how we get from breakpoint to breakpoint   debug gt  next break in misc debug js 1  gt  1  function  exports  require  module    filename    dirname    let a   5    2 debugger    3 debug gt  next break in misc debug js 2   1  function  exports  require  module    filename    dirname    let a   5   gt  2 debugger    3   4 a    2  debug gt  next break in misc debug js 4   2 debugger    3  gt  4 a    2    5 debugger    6   What we may do now is we may check the variable values at this point by writing repl command  This will allow you to write variable name and see its value   debug gt  repl Press Ctrl   C to leave debug repl  gt  a 5  gt  b undefined  gt  c undefined  gt    You may see that we have a   5 at this moment and b and c are undefined   Of course for more complex debugging you may want to use some external tools  IDE  browser   You may read more here

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The NetBeans IDE has had Node js support since version 8 1       lt           New Feature Highlights      Node js Application Development         New Node js project wizard   New Node js Express wizard   Enhanced JavaScript Editor   New support for running Node js applications   New support for debugging Node js applications           lt        Additional references    NetBeans Wiki   NewAndNoteworthyNB81  Node js Express App in NetBeans IDE  Geertjan-Oracle

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You can try-catch errors    function yourFunc       try            YOUR CODE HERE     catch  err          console error err message          err trace           Error message and error trace will give you infomation you need to identify and correct run-time bugs

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Visual Studio Code will work for us in debugging

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Use Webstorm  It s perfect for debugging Node js applications  It has a built-in debugger  Check out the docs here  https   www jetbrains com help webstorm 2016 1 running-and-debugging-node-js html

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Theseus is a project by Adobe research which lets you debug your Node js code in their Open Source editor Brackets  It has some interesting features like real-time code coverage  retroactive inspection  asynchronous call tree

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Using Chrome Version 67 0 3396 62      Run node app       node --inspect-brk 0 0 0 0 9229 server js server js filename     Browse your app in chrome e g   localhost port  Open DevTools  Click the the node icon beside the responsive device icon      There will be another DevTools window that will pop out specifically for debugging node app

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If you need a powerful logging library for Node js  Tracer https   github com baryon tracer is a better choice   It outputs log messages with a timestamp  file name  method name  line number  path or call stack  support color console  and support database  file  stream transport easily  I am the author

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Node has its own built in GUI debugger as of version 6 3  using Chrome s DevTools     Simply pass the inspector flag and you ll be provided with a URL to the inspector   node --inspect server js   You can also break on the first line by passing --inspect-brk instead

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Start your node process with --inspect flag   node --inspect index js  and then Open chrome   inspect in chrome  Click the  Open dedicated DevTools for Node  link or install this chrome extension for easily opening chrome DevTools   For more info refer to this link

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Enable VS Code preference Auto Attach from File -  Preferences -  Settings -  Search for Auto Attach and set it On  open the console  ctrl      or from the menu Terminal -  New Terminal  set a breakpoint in your code  type in the command  node --inspect  lt name of your file gt    This will start the debugging and the VS Code will show the Debug menu   Note  Auto Attach is also very helpful if you are working with node cluster worker threads

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There are many possibilities      node includes a debugging utility node-inspector Code editors   IDEs  see debug instructions for one of the following    Atom   VSCode Webstorm and more    Debug support is often implemented using the v8 Debugging Protocol or the newer Chrome Debugging Protocol

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There are may ways to debug Node JS application as follows   1  Install devtool and start application with it  npm install devtool -g --save devtool server js   this will open in chrome developer mode so you can put a debugger point and test   2  debug with node-inspector   node-inspector   3  debug with --debug  node --debug app js

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Another option not mentioned in other answers is using a tool called Rookout  It s used to debug and get data from both local and remote apps  We use it in our production environment to aggregate data to other services - saves us a lot of headaches and hardcoded logging

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If you are using the Atom IDE  you can install the node-debugger package

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Debugging   Joyent s Guide Debugger Node Inspector Visual Studio Code Cloud9 Brackets   Profiling   node --prof   app js node --prof-process   the-generated-log-file   Heapdumps   node-heapdump with Chrome Developer Tools   Flamegraphs   0x jam3 devtool then Chrome Developer Tools Flame Charts Dtrace and StackVis     Only supported on SmartOS clinicjs   Tracing   Interactive Stack Traces with TraceGL   Logging  Libraries that output debugging information   Caterpillar Tracer scribbles   Libraries that enhance stack trace information   Longjohn   Benchmarking   Apache Bench  ab -n 100000 -c 1 http   127 0 0 1 9778  wrk   Other   Trace Vantage Bugger Google Tracing Framework Paul Irish s Guide   Legacy  These use to work but are no longer maintained or no longer applicable to modern node versions    https   github com bnoordhuis node-profiler - replaced by built-in debugging https   github com c4milo node-webkit-agent - replaced by node inspector https   nodetime com  - defunct

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A quick-and-dirty way to debug small Node js scripts with your favorite browser debugger would be to use browserify  Note that this approach doesn t work with any applications which require native I O libraries  but it is good enough for most small scripts     npm install -g browserify   Now move all your var x   requires  x   calls into a requires js file and run     browserify requires js -s window -o bundle js    The downside here is that you either have to move or comment the requires in all your files    Include the bundle js in an HTML file like so    lt script type  text javascript  src  bundle js  gt  lt  script gt    Now load the file in your browser and press F12 and viola  debug in browser

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There is a lot of ways to debug but I prefer and I use builtin debugger by node js   app js file  var fs   require  fs     fs readFile  test txt    utf8   function  err  data     debugger   if  err  throw err   console log data         command  node debug app js

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