n-filter.ts

const isInDirectory = (directories: string[]) => (filePath: string) =>
  directories.some(filePath.startsWith.bind(filePath))

const isExpectedExtension = (extensions: string[]) => (extension: string) =>
  extensions.some(is(extension))

// ...

const shouldPathBeIgnored = isInDirectory(ignoredDirectories)
const shouldPathBeWatched = isInDirectory(watchedDirectories)
const shouldExtensionBeWatched = isExpectedExtension(extensions)

// ...

fromEvent(chokidar.watch(process.cwd()), 'all').pipe(
  // ...
  filter(
    ({ filePath, extension }) =>
      (shouldWatchEveryDirectory || shouldPathBeWatched(filePath)) &&
      (shouldWatchEveryExtension || shouldExtensionBeWatched(extension)) &&
      not(shouldPathBeIgnored(filePath)),
  ),
  // ...
)