Watch for changes on filename. The callback listener will be called each time the file is accessed.

The options argument may be omitted. If provided, it should be an object. The options object may contain a boolean named persistent that indicates whether the process should continue to run as long as files are being watched. The options object may specify an interval property indicating how often the target should be polled in milliseconds.

The listener gets two arguments the current stat object and the previous stat object:

import { watchFile } from 'node:fs';

watchFile('message.text', (curr, prev) => {
  console.log(`the current mtime is: ${curr.mtime}`);
  console.log(`the previous mtime was: ${prev.mtime}`);
});

These stat objects are instances of fs.Stat. If the bigint option is true, the numeric values in these objects are specified as BigInts.

To be notified when the file was modified, not just accessed, it is necessary to compare curr.mtimeMs and prev.mtimeMs.

When an fs.watchFile operation results in an ENOENT error, it will invoke the listener once, with all the fields zeroed (or, for dates, the Unix Epoch). If the file is created later on, the listener will be called again, with the latest stat objects. This is a change in functionality since v0.10.

Using watch is more efficient than fs.watchFile and fs.unwatchFile. fs.watch should be used instead of fs.watchFile and fs.unwatchFile when possible.

When a file being watched by fs.watchFile() disappears and reappears, then the contents of previous in the second callback event (the file's reappearance) will be the same as the contents of previous in the first callback event (its disappearance).

This happens when:

• the file is deleted, followed by a restore
• the file is renamed and then renamed a second time back to its original name