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What does fs.watch() do?

Node.jsJunior~1 min read
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In Node.js, fs.watch() is used to track changes in the filesystem, it "watches" files or folders and calls a callback when something changes.

Signature

javascript
fs.watch(filename[, options][, listener])

Arguments:

  • filename, the path to the file or directory to watch.
  • options (optional):
    • persistent: true by default, if false, the watch won't keep the Node.js process "alive";
    • recursive: true watches all subdirectories recursively (macOS and Windows only);
    • encoding: the encoding for filenames ('utf8' by default).
  • listener, the callback invoked on changes.

Example

javascript
const fs = require('fs'); fs.watch('./data', (eventType, filename) => { console.log(`Event: ${eventType}`); console.log(`File changed: ${filename}`); });

Possible eventType values:

  • "rename", a file was renamed, created, or deleted;
  • "change", a file was modified (for example, its content was updated).

A practical example

Watching a config file for changes:

javascript
fs.watch('config.json', (eventType) => { if (eventType === 'change') { console.log('The config file was updated! Reloading settings...'); // For example, re-read the JSON here } });

Quirks and limitations

  1. Behavior depends on the OS Node.js uses the platform's native APIs:
  • inotify on Linux,
  • FSEvents on macOS,
  • ReadDirectoryChangesW on Windows. So events can differ slightly across platforms.
  1. Not every change is guaranteed to be caught, especially when many files change at once.
  2. fs.watch() doesn't read the file's content, it only reports that something changed.
  3. For more reliable watching (with buffering and handling of missed events), the chokidar library is a better choice, it's cross-platform and more stable.

Summary

fs.watch() is Node.js's built-in function for watching files and folders for changes. It calls a callback on "change" or "rename" events, and it's useful for things like:

  • automatically reloading configuration,
  • rebuilding a project when files change,
  • monitoring logs or directories.

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