How to restart a Node.js app with watch mode

Node.js watch mode keeps a local development process tied to the files it loads, so a source edit restarts the app without adding nodemon or another watcher package. It is useful when testing small services, scripts, or framework-free prototypes where the important signal is the process starting again after a changed module is saved.

The built-in --watch flag watches the entry point and imported modules by default. Adding --watch-preserve-output keeps previous console output visible across restarts, which makes the restart line and the next startup message easier to inspect while developing.

A minimal HTTP server and an imported message module provide a visible check. The first request returns one value, the saved module edit restarts Node.js, and the next request returns the changed value. Keep watch mode for local development; production restarts, crash recovery, and boot persistence belong to a process manager or service supervisor.

Steps to restart a Node.js app with watch mode:

Create a message module for the app to import.
```
$ cat > message.mjs <<'EOF'
export const message = 'ready v1';
EOF
```
The .mjs extension keeps the sample in ES modules mode without adding a /package.json file.

Create a small HTTP server that uses the imported value.

$ cat > server.mjs <<'EOF'
import http from 'node:http';
import { message } from './message.mjs';

const server = http.createServer((req, res) => {
  res.end(`${message}\n`);
});

server.listen(3000, () => {
  console.log('server listening on http://127.0.0.1:3000');
});
EOF

Use another local port if 3000 is already assigned to a different development service.

Start the app with Node.js watch mode.
```
$ node --watch --watch-preserve-output server.mjs
server listening on http://127.0.0.1:3000
```
Plain --watch works on Linux, macOS, and Windows. Use --watch-path only on macOS and Windows when explicit watched directories are needed.
Request the app before changing the watched module.
```
$ curl -sS http://127.0.0.1:3000/
ready v1
```

Replace the imported message module with a changed value.

$ cat > message.mjs <<'EOF'
export const message = 'ready v2';
EOF

Check the watch-mode terminal after saving the module.
```
server listening on http://127.0.0.1:3000
Restarting 'server.mjs'
server listening on http://127.0.0.1:3000
```
The restart line appears in the terminal where node --watch is still running.
Request the app again after Node.js restarts it.
```
$ curl -sS http://127.0.0.1:3000/
ready v2
```
Stop the watch-mode process with Ctrl-C.
Remove the sample files when they are no longer needed.
```
$ rm server.mjs message.mjs
```