Monitoring and Waiting for File Changes in Node.js

This article explains how to monitor file changes and trigger actions using Node.js’s standard library “fs.watch”.

The following is sample code from the official documentation File system | Node.js v16.1.0 Documentation with some modifications.

const fs = require('fs');
fs.watch('./file.txt', (eventType, filename) => {
  console.log(`event type is: ${eventType}`);
  if (filename) {
    console.log(`filename provided: ${filename}`);
  } else {
    console.log('filename not provided');
  }
});

This monitors the file ”./file.txt” in the current directory and logs when changes occur.

For this article, I’ll implement a sample that performs an action when the file changes - specifically, waiting for file changes and continuing the script when a change occurs. Here’s the code example. I wrote it to be as simple as possible, eliminating unnecessary code and implementing only the minimum functionality.

(async function(){

    await (new Promise((resolve) => {
        fs.watch('./file', (e, f) => { resolve() })
    })); // wait for file change
    console.log("changed.");

})();

Since fs.watch doesn’t have a synchronous version like filereadsync, we implement it by combining it with a promise. We wait for the promise with “await”, and when the ./file changes, we execute resolve() to exit the promise and continue to the console.log.

In this article, we implemented a simple file change waiting mechanism using built-in Node.js libraries. However, if you want to create a command-line tool with this functionality or build something more complex, I recommend checking out other existing libraries and command-line tools, such as bash’s “inotifywait” or Node.js library “nodemon”.