How to get docker mapped ports from node.js application?

Question

I would like to get mapped port from inside node.js application.

ex.

docker-compose:

my-app:
    build:
        context: ./my-app
           


        

Answer 1

You can use docker port for this.

docker port my-app 80

That will include the listener IP. If you need to strip that off, you can do that with the shell:

docker port my-app 80 | cut -f2 -d:

Unfortunately, this will only work with access to the docker socket. I wouldn't recommend mounting that socket for just this level of access inside your container.

Typically, most people solve this by passing a variable to their app and controlling what port is published in their docker-compose file. E.g.:

my-app:
    build:
        context: ./my-app
    ports:
        - "8080:80"     
    environment:
        - PUBLISHED_PORT=8080
    volumes:
        - /var/run/docker.sock:/tmp/docker.sock
    networks:
        - private

Then the app would look at the environment variable to reconfigure itself.