fs: is fs.unwatchFile() available on Windows?

[vsemozhetbyt]

• Version: v4 – master
• Platform: Windows 7 x64
• Subsystem: fs

1. Create a script:

'use strict';

const fs = require('fs');

fs.watchFile(__filename, { interval: 1000 }, (current, previous) => {
  console.log(current.size, previous.size);
  fs.unwatchFile(__filename, () => { console.log('File unwatched'); });
  console.log('unwatchFile() called');
});

2. Launch it, then change and save several times.

3. Expected output:

275 274
unwatchFile() called
File unwatched
[exit]

Real output

275 274
unwatchFile() called
276 275
unwatchFile() called
277 276
unwatchFile() called
[.., no exit]

I cannot find any caveats in fs doc about this. Is there any?

[vsemozhetbyt]

This works as intended:

'use strict';

const fs = require('fs');

const watcher = fs.watch(__filename, (eventType) => {
  console.log(eventType);
  watcher.close();
  console.log('watcher.close() called');
});

change
watcher.close() called
[exit]

[richardlau]

The listener argument passed to fs.unwatchFile should be a callback function that was previously registered with fs.watchFile and is not expected to be called as part of the invocation of fs.unwatchFile.

[richardlau]

To clarify, in your example the fs.unwatchFile call does not remove the listener registered with the fs.watchFile call so never exits because there is still an active registered callback.

[vsemozhetbyt]

@richardlau Thank you! This do work as intended:

'use strict';

const fs = require('fs');

const listener = (current, previous) => {
  console.log(current.size, previous.size);
  fs.unwatchFile(__filename, listener);
  console.log('unwatchFile() called');
}

fs.watchFile(__filename, { interval: 1000 }, listener);

I was misguided by the listener signature in last descriptions:

https://nodejs.org/download/nightly/v10.0.0-nightly20180122e7ff00d0c5/docs/api/fs.html#fs_fs_unwatchfile_filename_listener

Is this signature intended? Added here: #13424