Debug a NodeJs app in Docker

This project shows how to configure your dev environment to be able to debug a NodeJs application deployed in a Docker container.

Pre-Requisites

Install the following software in your dev machine:

• Visual Studio Code
• Docker

Getting started

Download the project

$ git clone https://github.com/riccardomerlin/docker-debug-nodejs

Run the app in a docker container

$ cd docker-debug-nodejs
$ docker build --tag docker-debug-nodejs:latest .
$ docker run --publish 3000:3000  docker-debug-nodejs:latest

Open a browser to http://localhost:3000 and the page should display Hello Docker World!.

Run the app in debug mode in a Docker container

1. Run the following command from the shell to start a new container instance with debug port (9222) exposed

   $ docker run -p 3000:3000 -p 9222:9222 --entrypoint=/bin/bash docker-debug-nodejs:latest -c "npm run debug"

   or alternatively

   start the container using docker-compose

   $ docker-compose up

2. Open up /src/index.js in VS Code and set a break point at this line

   res.end('Hello Docker World!')

3. Open the Debug panel and select Docker: Attach to Node configuration to attach the debugger. VS Code should display the debugger attached in the status bar at the very bottom of the window.

4. Open the browser and go to http://localhost:3000, you will see the debugger in VS Code stopping in the break point set in the previous step.

Hot-reload

Note: it works only when running the container with docker-compose

Edit the following line in /src/index.js

res.end('Hello Docker World!')

to

res.end('Hello Docker World [Edited]!')

and save the file.

In the terminal you should see node restarting and the debugger should automatically re-attach.

Go back to the browser and refresh the page, the debugger is still stopping in the same break-point. Release the debugger and the page now displays Hello Docker World [Edited]!.

Things to notice

launch.json

• "remoteRoot", it has to specify the execution folder in the container in order to make the debugger able to set the break point correctly.
• "restart", set it to true to support hot changes. This flag allows the debugger re-attaching after node is restarted.

docker-compose

• volumes, maps a folder in the host to a folder in the container (arguments separated by :)
• ports, maps container ports with host ports