public async init(): Promise<FSWatcher> {
   try {
     logger.info('Starting watcher');
     return this.fswatcher.on('add', this.onAdd.bind(this));
   } catch (err) {
     logger.error(err);
     throw err;
   }
 }

 public constructor(
   private serializer: Serializer,
   private path: string,
   private extension: string,
   watcher?: FSWatcher
 ) {
   super();
   if (watcher) {
     watcher
       .on('add',    filePath => this.onFileUpdated(filePath))
       .on('change', filePath => this.onFileUpdated(filePath))
       .on('unlink', filePath => this.onFileRemoved(filePath))
       .add(path);
   }
 }

it('watch()', () => {
    let watcher: FSWatcher;

    return fs.watch(tmpDir).then(watcher_ => {
        watcher = watcher_;

        return new Promise((resolve, reject) => {
            const path = join(tmpDir, 'test.txt');

            watcher.on('add', (path_) => {
                path_.should.eql(path);
                resolve();
            });

            fs.writeFile(path, 'test').catch(reject);
        });
    }).finally(() => {
        if (watcher) watcher.close();
    });
});

 public constructor(watcher: FSWatcher) {
   watcher
     .on('add',    path => log(chalk.cyan('ADD ') + path))
     .on('change', path => log(chalk.cyan('CHG ') + path))
     .on('unlink', path => log(chalk.cyan('REM ') + path));
 }

        return new Promise((resolve, reject) => {
            const path = join(tmpDir, 'test.txt');

            watcher.on('add', (path_) => {
                path_.should.eql(path);
                resolve();
            });

            fs.writeFile(path, 'test').catch(reject);
        });

 }).finally(() => {
     if (watcher) watcher.close();
 });