Chokidar

A neat wrapper around node.js fs.watch / fs.watchFile / FSEvents.

Why?

Node.js fs.watch:

Doesn't report filenames on MacOS.
Doesn't report events at all when using editors like Sublime on MacOS.
Often reports events twice.
Emits most changes as rename.
Has a lot of other issues
Does not provide an easy way to recursively watch file trees.

Node.js fs.watchFile:

Almost as bad at event handling.
Also does not provide any recursive watching.
Results in high CPU utilization.

Chokidar resolves these problems.

Initially made for Brunch (an ultra-swift web app build tool), it is now used in
gulp,
karma,
PM2,
browserify,
webpack,
BrowserSync,
Microsoft's Visual Studio Code,
and many others.
It has proven itself in production environments.

How?

Chokidar does still rely on the Node.js core fs module, but when using
fs.watch and fs.watchFile for watching, it normalizes the events it
receives, often checking for truth by getting file stats and/or dir contents.

On MacOS, chokidar by default uses a native extension exposing the Darwin
FSEvents API. This provides very efficient recursive watching compared with
implementations like kqueue available on most *nix platforms. Chokidar still
does have to do some work to normalize the events received that way as well.

On other platforms, the fs.watch-based implementation is the default, which
avoids polling and keeps CPU usage down. Be advised that chokidar will initiate
watchers recursively for everything within scope of the paths that have been
specified, so be judicious about not wasting system resources by watching much
more than needed.

Getting started

Install with npm:

npm install chokidar

Then require and use it in your code:

var chokidar = require('chokidar');

// One-liner for current directory, ignores .dotfiles
chokidar.watch('.', {ignored: /(^|[\/\\])\../}).on('all', (event, path) => {
  console.log(event, path);
});

// Example of a more typical implementation structure:

// Initialize watcher.
var watcher = chokidar.watch('file, dir, glob, or array', {
  ignored: /(^|[\/\\])\../,
  persistent: true
});

// Something to use when events are received.
var log = console.log.bind(console);
// Add event listeners.
watcher
  .on('add', path => log(`File ${path} has been added`))
  .on('change', path => log(`File ${path} has been changed`))
  .on('unlink', path => log(`File ${path} has been removed`));

// More possible events.
watcher
  .on('addDir', path => log(`Directory ${path} has been added`))
  .on('unlinkDir', path => log(`Directory ${path} has been removed`))
  .on('error', error => log(`Watcher error: ${error}`))
  .on('ready', () => log('Initial scan complete. Ready for changes'))
  .on('raw', (event, path, details) => {
    log('Raw event info:', event, path, details);
  });

// 'add', 'addDir' and 'change' events also receive stat() results as second
// argument when available: http://nodejs.org/api/fs.html#fs_class_fs_stats
watcher.on('change', (path, stats) => {
  if (stats) console.log(`File ${path} changed size to ${stats.size}`);
});

// Watch new files.
watcher.add('new-file');
watcher.add(['new-file-2', 'new-file-3', '**/other-file*']);

// Get list of actual paths being watched on the filesystem
var watchedPaths = watcher.getWatched();

// Un-watch some files.
watcher.unwatch('new-file*');

// Stop watching.
watcher.close();

// Full list of options. See below for descriptions. (do not use this example)
chokidar.watch('file', {
  persistent: true,

  ignored: '*.txt',
  ignoreInitial: false,
  followSymlinks: true,
  cwd: '.',
  disableGlobbing: false,

  usePolling: true,
  interval: 100,
  binaryInterval: 300,
  alwaysStat: false,
  depth: 99,
  awaitWriteFinish: {
    stabilityThreshold: 2000,
    pollInterval: 100
  },

  ignorePermissionErrors: false,
  atomic: true // or a custom 'atomicity delay', in milliseconds (default 100)
});

API

chokidar.watch(paths, [options])

paths (string or array of strings). Paths to files, dirs to be watched
recursively, or glob patterns.
options (object) Options object as defined below:

Persistence

persistent (default: true). Indicates whether the process
should continue to run as long as files are being watched. If set to
false when using fsevents to watch, no more events will be emitted
after ready, even if the process continues to run.

Path filtering

ignored (anymatch-compatible definition)
Defines files/paths to be ignored. The whole relative or absolute path is
tested, not just filename. If a function with two arguments is provided, it
gets called twice per path - once with a single argument (the path), second
time with two arguments (the path and the
fs.Stats
object of that path).
ignoreInitial (default: false). If set to false then add/addDir events are also emitted for matching paths while
instantiating the watching as chokidar discovers these file paths (before the ready event).
followSymlinks (default: true). When false, only the
symlinks themselves will be watched for changes instead of following
the link references and bubbling events through the link's path.
cwd (no default). The base directory from which watch paths are to be
derived. Paths emitted with events will be relative to this.
disableGlobbing (default: false). If set to true then the strings passed to .watch() and .add() are treated as
literal path names, even if they look like globs.

Performance

usePolling (default: false).
Whether to use fs.watchFile (backed by polling), or fs.watch. If polling
leads to high CPU utilization, consider setting this to false. It is
typically necessary to set this to true to successfully watch files over
a network, and it may be necessary to successfully watch files in other
non-standard situations. Setting to true explicitly on MacOS overrides the
useFsEvents default. You may also set the CHOKIDAR_USEPOLLING env variable
to true (1) or false (0) in order to override this option.
Polling-specific settings (effective when usePolling: true)
interval (default: 100). Interval of file system polling. You may also
set the CHOKIDAR_INTERVAL env variable to override this option.
binaryInterval (default: 300). Interval of file system
polling for binary files.
(see list of binary extensions)
useFsEvents (default: true on MacOS). Whether to use the
fsevents watching interface if available. When set to true explicitly
and fsevents is available this supercedes the usePolling setting. When
set to false on MacOS, usePolling: true becomes the default.
alwaysStat (default: false). If relying upon the
fs.Stats
object that may get passed with add, addDir, and change events, set
this to true to ensure it is provided even in cases where it wasn't
already available from the underlying watch events.
depth (default: undefined). If set, limits how many levels of
subdirectories will be traversed.
awaitWriteFinish (default: false).
By default, the add event will fire when a file first appears on disk, before
the entire file has been written. Furthermore, in some cases some change
events will be emitted while the file is being written. In some cases,
especially when watching for large files there will be a need to wait for the
write operation to finish before responding to a file creation or modification.
Setting awaitWriteFinish to true (or a truthy value) will poll file size,
holding its add and change events until the size does not change for a
configurable amount of time. The appropriate duration setting is heavily
dependent on the OS and hardware. For accurate detection this parameter should
be relatively high, making file watching much less responsive.
Use with caution.
options.awaitWriteFinish can be set to an object in order to adjust
timing params:
awaitWriteFinish.stabilityThreshold (default: 2000). Amount of time in
milliseconds for a file size to remain constant before emitting its event.
awaitWriteFinish.pollInterval (default: 100). File size polling interval.

Errors

ignorePermissionErrors (default: false). Indicates whether to watch files
that don't have read permissions if possible. If watching fails due to EPERM
or EACCES with this set to true, the errors will be suppressed silently.
atomic (default: true if useFsEvents and usePolling are false).
Automatically filters out artifacts that occur when using editors that use
"atomic writes" instead of writing directly to the source file. If a file is
re-added within 100 ms of being deleted, Chokidar emits a change event
rather than unlink then add. If the default of 100 ms does not work well
for you, you can override it by setting atomic to a custom value, in
milliseconds.

Methods & Events

chokidar.watch() produces an instance of FSWatcher. Methods of FSWatcher:

.add(path / paths): Add files, directories, or glob patterns for tracking.
Takes an array of strings or just one string.
.on(event, callback): Listen for an FS event.
Available events: add, addDir, change, unlink, unlinkDir, ready,
raw, error.
Additionally all is available which gets emitted with the underlying event
name and path for every event other than ready, raw, and error.
.unwatch(path / paths): Stop watching files, directories, or glob patterns.
Takes an array of strings or just one string.
.close(): Removes all listeners from watched files.
.getWatched(): Returns an object representing all the paths on the file
system being watched by this FSWatcher instance. The object's keys are all the
directories (using absolute paths unless the cwd option was used), and the
values are arrays of the names of the items contained in each directory.

CLI

If you need a CLI interface for your file watching, check out
chokidar-cli, allowing you to
execute a command on each change, or get a stdio stream of change events.

Install Troubleshooting

npm WARN optional dep failed, continuing fsevents@n.n.n
This message is normal part of how npm handles optional dependencies and is
not indicative of a problem. Even if accompanied by other related error messages,
Chokidar should function properly.
ERR! stack Error: Python executable "python" is v3.4.1, which is not supported by gyp.
You should be able to resolve this by installing python 2.7 and running:
npm config set python python2.7
gyp ERR! stack Error: not found: make
On Mac, install the XCode command-line tools

License

The MIT License (MIT)

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the “Software”), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.