Watchman is an open-source file watching service by Meta that triggers actions when matching files change.
Getting Started
# Add ./src directory to the watch list
watchman watch ./src
# Add a trigger named 'retest' that executes npm test when any *.js file in ./src changes
watchman -- trigger ./src retest '*.js' -- npm test
Managing Watches
watchman watch ~/src # Watch ~/src
watchman watch-list # List all active watches
watchman watch-del ~/src # Remove watch on ~/src