Assuming you are on a Linux system, let's create a temp directory.

mkdir /tmp/client_server

And move into the temp directory.

cd /tmp/client_server

Use the npm init command to create package.json.

npm init -y

Let's update package.json to have "type": "module" since this will be an ES Module.

{
  "type": "module",
  "name": "bar",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

Create a file named Dockerfile that contains the following.

FROM node:18-alpine
WORKDIR /src
COPY . .
RUN npm install
EXPOSE 12345

You should now have the following files in your present working directory.

├── Dockerfile
├── package.json

On your Docker server, use the docker build command to create the Node.js Express image.

sudo docker build --file Dockerfile --tag nodejs-express:18-alpine .

The docker images command should return something like this.

~]$ sudo docker images
REPOSITORY        TAG          IMAGE ID       CREATED         SIZE
nodejs-express    18-alpine    824127ec51d1   1 minute ago    189MB

Now let's create two apps, the Node.js client app that will connect to the Node.js server app.

Create /tmp/server.js that contains the following.

import net from "net";
import os from "os";

const server = net.createServer();
const timestamp = () => `[${new Date().toUTCString()}]`
const port = 12345
const host = os.hostname();

server.on("connection", (socket) => {
    console.log(`${timestamp()} successfully connected to server.js`)
})

server.listen(port, host, () => {
    console.log(`${timestamp()} server listening for connections at ${host}:${port}`);
})

The docker run command can be used to create and start a Docker container from the Node.js Express image with server.js and then use the node server.js command to run server.js.

sudo docker run \
--name nodejs-server \
--publish 0.0.0.0:12345:12345 \
--detach \
--volume /tmp/server.js:/src/server.js \
nodejs-express:18-alpine \
node server.js

The docker container ls command should show the Node.js server container is up and running.

~]$ sudo docker container ls
CONTAINER ID   IMAGE                        COMMAND                  CREATED         STATUS          PORTS                            NAMES
e901cdaafafa   nodejs-express:18-alpine     "docker-entrypoint.s…"   4 seconds ago   Up 3 seconds    0.0.0.0:12345->12345/tcp         nodejs-server

The docker logs command should return something like this.

~]$ sudo docker logs nodejs-server
[Sun, 01 Sep 2024 12:47:57 GMT] server listening for connections at e901cdaafafa:12345

Let's create /tmp/client.js with the following markup using the hostname of the Docker host. In this example, the Docker host is running on an Amazon Web Services (AWS) server with hostname ec2-11-12-13-14.compute-1.amazonaws.com.

import net from "net";
import os from "os";

const timestamp = () => `[${new Date().toUTCString()}]`
const port = 12345
const host = "ec2-11-12-13-14.compute-1.amazonaws.com";

net.createConnection({host: host, port: port},() => {
    console.log(`${timestamp()} successfully connected to server.js`);
})

The docker run command can be used to create and start a Docker container from the Node.js Express image with client.js and then use the node client.js command to run client.js.

sudo docker run \
--name nodejs-client \
--publish 0.0.0.0:22222:22222 \
--detach \
--volume /tmp/client.js:/src/client.js \
nodejs-express:18-alpine \
node client.js

The docker container ls command should show the container is up and running.

~]$ sudo docker container ls
CONTAINER ID   IMAGE                        COMMAND                  CREATED         STATUS          PORTS             NAMES
e901cdaafafa   nodejs-express:18-alpine     "docker-entrypoint.s…"   4 seconds ago   Up 3 seconds    12345/tcp         nodejs-clinet

The docker logs command should return something like this.

~]$ sudo docker logs nodejs-client
[Sun, 01 Sep 2024 12:56:37 GMT] successfully connected to server.js