Supercharge your debugging experience for Node.js

Debugging with Node

You see this, what do you do?

hypothetical: You might check every line of code and try to find a typo. No typo found… next you might start using console.logs to see the output for code, which is a pain. Surely there are better methods out there?

Debugging server side javascript code with console logs is archaic.

Console logs just simply aren’t enough since it requires us to go line by line in a very slow manner whilst restarting our server each time. Console logs also clutter our codebase with unnecessary code and removing them requires small effort.

Exhibit A: How much energy does it take to spot the console log among the dummy data and other processes?

uh-oh, console cluttered. Where’s my console log?

Very taunting. It doesn’t have to be this way.

Much more efficient way of doing things would be debugging with breakpoints. With breakpoints we can step over the call stack and event loop to diagnose the problem.

Philip Roberts: What the heck is the event loop anyway? | JSConf EU

You get the point. Time to explore other options. What are the options? I’m glad you asked!

By the way there’s a very tasty treat at the end of the article — just sayin.

Node has a built-in inspector!

Yes, that’s right. Node actually has a built-in inspector. The inspector uses the v8 debugger protocol viewer under the hood.

Let me show you — let’s start a new node project.

starting our project and installing express and nodemon
server.js

Now we have a basic express server. But instead of using the node server.js command the traditional way, we’re adding a extra flag to our node command, the inspector.

--inspect tells Node to expose the new debugging protocol.

Once we start our server with the inspector, here’s what the console outputs.

Sweet, it worked. Open the Chrome at http://localhost:3000 with the dev tools. Notice something extra?

Notice the Node element next to the mobile inspector

Woot, node inside our browser? Yes indeed! We still consume our app like before — localhost:3000

http://0.0.0.0:9229/ port is for the DevTools to consume.

Inspecting server side code inside our browser

What does Node Inspector do?

Node Inspector lets you to use the DevTools user interface with the native Node debugger. DevTools can now connect directly to the Node process!

If you know how to get this working on Firefox or Safari, let us know in the comments/discussion.


Using the debugger

Using the chrome debugger is very much the same like using the debugger for client side code. You set breakpoints, execute the code, step over the breakpoints and find the bug.

Using breakpoints to step over our code

Can you imagine how useful this can be when we have an error inside a big controller? We have access to the debugger, call stack, scopes, local variables, global variables, etc. We have so many sharp tools to use!

Taking it to another level

We’re not done yet. Yesterday (20 July, 2018) was a very special day for debugging node. Why?

Google Chrome labs team open sourced their advanced debugging tool — ndb!

ndb is an improved debugging experience for Node.js, enabled by Chrome DevTools

Ndb is a sweet tool for debugging. Let’s give it a go.

Installing ndb

npm install -g ndb

It’s like any other npm package, very simple to use and install.

Usage

To use ndb, we just have to prefix our start script with ndb.

appending ndb to our start script

That’s all! Let’s restart our server — notice we’re using nodemon — just like any traditional project.

Woot! We have a new chrome instance for the sole purpose of debugging. How cool is that? Epic!

We even have access to the node process global object — which is the window object for node.

Hacker news discussion about the new debugger — check it out!

Thanks for reading, stay awesome! ❤