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Stop only using console.log and use the built-in node debugger

nyxtom profile image Tom Holloway 🏕 Updated on ・3 min read

Fun fact: you don't necessarily have to just use console.log to debug your application, node has a built-in debugger that works with DevTools.

What's great, is that this feature has been around since Node 6.3 (that's 4 years ago!). Prior to this there was a package that was around that let you do the same thing. Even still, you could use the console based debugger like many other languages let you. I'm going to show you how to use both.

Running with --inspect

You can run any node script with the following flag.

node --inspect index.js

But if you're like me, you probably have a more complicated setup that involves gulp or some other system that forks a number of processes. When you run nodemon with --inspect=5858 and your process ends up forking, the ports will increment 1 by 1. If this happens, and your debugging port is 5858 then the debug ports for the other processes will be 5859, 5860...etc.

Chrome Inspector

Open up Chrome to chrome://inspect and you may notice the link with the following:

Open dedicated DevTools for Node

Click on that link and you'll get a DevTools inspector. All you need to do is go to the Connection tab and add the various debug ports to the connection list. For me, this was localhost:5858, localhost:5859, localhost:5860, 'localhost:5861', localhost:5862.

DevTools Connections

Once you've done that you are good to go. You may notice in your terminal that a debugger was attached.

Chrome Node Inspector

Features

The great thing about the node inspector is that it comes with loads of features:

  • Breakpoint debugging, stepping and blackboxing
  • Source maps for transpiled code
  • LiveEdit w/ hotspot evaluation
  • Console evaluation
  • Profiling and sampling
  • Heap snapshots, allocation, memory profiling
  • Async stacks/promises

There you go!

Bonus If you ssh you can forward your debugging for a remote debugging session if you're trying to debug something running on a server.

ssh -L 9221:localhost:9229 user@remote.example.com

For more details check out the documentation guide!

Debugger Command Line

If you don't have access to chrome devtools and you can't do any ssh tunneling, you can also use the normal command line debugger. All you have to do is run:

node inspect 127.0.0.1:5858

You also have the option of attaching directly to a process id:

node inspect -p <process id>

Command Line Debugger

Good luck!

If you liked this article, please feel free to follow and check out my other posts on some other great tools in the Node.js ecosystem including

If you're looking for articles on linux and network diagnostics then check out:

Thanks!

Also feel free to follow me here and on twitter and github

Posted on by:

nyxtom profile

Tom Holloway 🏕

@nyxtom

Software Developer for as long as I can remember, building startups and projects. I've worked mostly in analytics, devops, and data science research. Running has proved useful so I enjoy doing it.

Discussion

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I know (and knew) that there is a node.js debugger, but there are still cases where inserting a console.log statement is the quickest and most practical way to know what's going on. So in my opinion the title of your article should rather be "Stop using console.log ONLY" :-)

 
 

I cannot shake this habit. Remove Console then yeah I would use debugger, sometimes I just want to dubug at a glance right in my IDE

 

Woke up this morning wondering if there was a better way that printing to the console. Wasn't even specifically searching for this, but I am sure glad I found it. Bookmarked and Shared!

 

I honestly did not know this even existed... Thanks! #LearnSomethingNewEveryDay

 

No problem! I put stuff like this as a reminder to myself that these sort of tools are there to help. Glad it helps!

 

This is really helpful for us. Do you know how can we attach debugger to serverless-framework's of lambda functions? I was facing annoying issue with lambda function and it's very hard to put console.log everywhere.

 

Thx! Very helpful info 👍