Chokidar

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

Why?

Node.js fs.watch:

Doesn't report filenames on OS X.
Doesn't report events at all when using editors like Sublime on OS X.
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 Mac OS X, 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 --save

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 OS X 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 OS X). 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 OS X, 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.

adesugbaa/chokidar