NodeJS by Example: Watch Mode

Watch Mode automatically restarts your Node.js application when files change. It's built into Node.js - no external tools needed.

Basic Watch Mode Run with: node --watch script.js Node.js will restart automatically when the file changes.

console.log('Server starting...');
console.log('Current time:', new Date().toISOString());

What Gets Watched By default, Node.js watches: - The main script file - All imported/required modules Example server that demonstrates watch mode

import { createServer } from 'node:http';

const PORT = process.env.PORT || 3000;

const server = createServer((req, res) => {
  // Change this message and save - server auto-restarts
  const message = 'Hello from Watch Mode!';
  
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end(message + '\n');
});

server.listen(PORT, () => {
  console.log(`Server running at http://localhost:${PORT}/`);
  console.log('Modify this file and save to see auto-restart');
});

Watch Specific Paths Use --watch-path to watch additional directories node --watch --watch-path=./src --watch-path=./config script.js

Preserving State Watch mode restarts the entire process, so state is lost. Use external storage (files, databases) to preserve state across restarts. To preserve: write to file or use a database

let requestCount = 0; // This resets on every restart

import { readFileSync, writeFileSync, existsSync } from 'node:fs';

function getPersistedCount() {
  try {
    if (existsSync('./count.txt')) {
      return parseInt(readFileSync('./count.txt', 'utf8')) || 0;
    }
  } catch {
    // Ignore errors
  }
  return 0;
}

function saveCount(count) {
  writeFileSync('./count.txt', String(count));
}

Watch Mode vs nodemon Built-in watch mode advantages: - No dependencies needed - Faster startup - Works with latest Node.js features nodemon advantages: - More configuration options - Can run non-Node.js scripts - Supports ignore patterns

Development Workflow Typical development setup in package.json: { "scripts": { "start": "node server.js", "dev": "node --watch server.js", "dev:debug": "node --watch --inspect server.js" } }

Combining with Other Flags Watch mode works with other Node.js flags node --watch --inspect server.js # With debugger node --watch --env-file=.env server.js # With env file node --watch --experimental-strip-types server.ts # With TypeScript

Watch Events The process receives events when files change

process.on('SIGTERM', () => {
  console.log('Shutting down gracefully...');
  server.close(() => {
    console.log('Server closed');
    process.exit(0);
  });
});

Tips for Watch Mode 1. Keep startup time fast - watch mode restarts frequently 2. Use environment variables for configuration 3. Handle SIGTERM for graceful shutdown 4. Use --watch-path for monorepos with multiple packages

Debugging with Watch Mode Combine --watch with --inspect for debugging node --watch --inspect server.js Then attach your debugger (VS Code, Chrome DevTools)

if (process.env.NODE_INSPECT) {
  console.log('Debugger attached');
  debugger; // Breakpoint here when debugging
}

Run in watch mode When you save the file:

$ node --watch watch-mode.js
# Server starting...
# Server running at http://localhost:3000/
# Modify this file and save to see auto-restart

# Restarting 'watch-mode.js'
# Server starting...
# Server running at http://localhost:3000/

Watch specific paths

$ node --watch --watch-path=./src server.js

Combine with debugging

$ node --watch --inspect server.js
# Debugger listening on ws://127.0.0.1:9229/...