Watch is a very simple tool that does one thing: run a command, and every time source files change in a directory, shutdown the current invocation gracefully and restart it.
deno -A jsr:@frontside/watch --command "npm start"
Watch will send SIGINT and SIGTERM to your command, and then wait until its stdout stream is closed, indicating that it has no further output. It will not attempt to start your command again until that has happened. This is important, because your process might be holding onto any number of resources that have to be safely released before exiting.
If you are running this command inside a git repository, it will only perform restarts on files that are under source control, or could be candidates for source control (not ignored).