Nodemon vs Node.js Process: Avoiding Duplicate Writes During Rapid Restarts

Answer

To stop duplicate log entries when Nodemon restarts the Node.js process quickly, configure Nodemon to debounce restarts, exclude the log directory, and ensure the application shuts down gracefully.

Likely explanation

Nodemon watches source files with chokidar and spawns a new child Node.js process on each change. If several file‑system events arrive in quick succession, the old process may still be writing to a log file when the new process starts, causing overlapping writes and duplicate lines.

Confirmed facts

• Nodemon sends SIGTERM to the old child process and starts the new one immediately; the old process may not have flushed its stdout/stderr or file handles before termination.
• Using --delay (or --legacy-watch) introduces a debounce period that lets the previous process exit before a new one is spawned.
• Adding an --ignore pattern for the log directory prevents Nodemon from triggering a restart when the application writes to its own log file.

Steps to prevent duplicate writes

1. Start Nodemon with a debounce delay that matches your edit‑save cadence, e.g. nodemon --delay 2.0 src/index.js.
2. Exclude the log folder from watching: nodemon --delay 2.0 --ignore 'logs/**' src/index.js.
3. In the application code, listen for the SIGTERM signal and flush/close any log streams before exiting:

const fs = require('fs');
const logStream = fs.createWriteStream('logs/app.log', { flags: 'a' });
process.on('SIGTERM', () => {
  logStream.end(() => process.exit(0));
});
// normal logging …
logStream.write(message + '\n');

If you cannot modify the application to handle SIGTERM, rely solely on the debounce and ignore options; increase the delay until duplicate lines disappear in your test.
Missing diagnostic detail
Can you modify the application to listen for SIGTERM and flush its log streams? Knowing this determines whether the graceful‑shutdown step is required or if the debounce/ignore approach alone will suffice.