webpack/lib/WatchIgnorePlugin.js

function WatchIgnorePlugin(paths) {
	this.paths = paths;
}

module.exports = WatchIgnorePlugin;

WatchIgnorePlugin.prototype.apply = function(compiler) {
	compiler.plugin("after-environment", function() {
		compiler.watchFileSystem = new IgnoringWatchFileSystem(compiler.watchFileSystem, this.paths);
	}.bind(this));
};

function IgnoringWatchFileSystem(wfs, paths) {
	this.wfs = wfs;
	this.paths = paths;
}

IgnoringWatchFileSystem.prototype.watch = function(files, dirs, missing, startTime, delay, callback, callbackUndelayed) {
	var ignored = function(path) {
		return this.paths.some(function(p) {
			return p instanceof RegExp ? p.test(path) : path.indexOf(p) === 0;
		});
	}.bind(this);

	var notIgnored = function(path) {
		return !ignored(path);
	};
	var ignoredFiles = files.filter(ignored);
	var ignoredDirs = dirs.filter(ignored);

	this.wfs.watch(files.filter(notIgnored), dirs.filter(notIgnored), missing, startTime, delay, function(err, filesModified, dirsModified, fileTimestamps, dirTimestamps) {
		if(err) return callback(err);

		ignoredFiles.forEach(function(path) {
			fileTimestamps[path] = 1;
		});

		ignoredDirs.forEach(function(path) {
			dirTimestamps[path] = 1;
		});

		callback(err, filesModified, dirsModified, fileTimestamps, dirTimestamps);
	}, callbackUndelayed);
};