09 — Wire app to Postgres using environment variables

Category: Docker Compose · Priority: High · Step: 10

Use DB_HOST=db (the service name), DB_PORT=5432. Never hardcode credentials — read them from the .env file.

🎯 What is this task?

Now you'll update your app so it actually reads from the Postgres database using environment variables. The key rule: never hardcode database credentials in your code. Always read them from process.env. This is how every real production app works.

🤖 AI Prompt

"Update my Node.js Express app (index.js) to connect to Postgres using the pg npm package. Read all connection details from environment variables: DB_HOST, DB_PORT, DB_NAME, DB_USER, DB_PASSWORD. Add a new route GET /db-test that runs a simple query SELECT NOW() and returns the result as JSON. Handle connection errors gracefully. Show the full updated index.js and the updated package.json."

📋 Step-by-step

Step 1 — Install the Postgres client library

npm install pg

This adds pg to your package.json dependencies.

Step 2 — Update index.js

const express = require('express');
const { Pool } = require('pg');

const app = express();
const PORT = process.env.PORT || 3000;

// Database connection — reads from environment variables
const pool = new Pool({
  host: process.env.DB_HOST,
  port: process.env.DB_PORT,
  database: process.env.DB_NAME,
  user: process.env.DB_USER,
  password: process.env.DB_PASSWORD,
});

app.get('/', (req, res) => {
  res.json({ message: 'Hello from Docker!' });
});

app.get('/health', (req, res) => {
  res.json({ status: 'ok' });
});

// New route to test DB connection
app.get('/db-test', async (req, res) => {
  try {
    const result = await pool.query('SELECT NOW() as current_time');
    res.json({
      status: 'connected',
      db_time: result.rows[0].current_time
    });
  } catch (error) {
    res.status(500).json({
      status: 'error',
      message: error.message
    });
  }
});

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

Step 3 — Rebuild and restart

Because you added a new npm package, you need to rebuild the Docker image:

docker compose down
docker compose up --build

The --build flag forces Docker to rebuild the image with the new pg dependency.

Step 4 — Test the database connection

Open http://localhost:3000/db-test in your browser. You should see something like:

{
  "status": "connected",
  "db_time": "2026-03-09T12:00:00.000Z"
}

If you see an error, check docker compose logs app to see what went wrong.

✅ Done when

• pg is in package.json dependencies
• index.js reads DB credentials from process.env
• http://localhost:3000/db-test returns { "status": "connected" }
• No credentials are hardcoded in the source code