Watchdog.ps1

# https://stackoverflow.com/questions/12319382/file-system-watcher-event-when-run-using-a-script#12323930

$watcher = new-object System.IO.FileSystemWatcher
$watcher.Path = '.\'
$watcher.Filter = '*.js'  # whatever you need
#$watcher.IncludeSubDirectories = $true  # if needed
$watcher.EnableRaisingEvents = $true

Register-ObjectEvent $watcher -EventName Changed -SourceIdentifier 'Watcher' -Action { param($sender, $eventArgs) 
   Write-Host "Detected change in files copying...";
   # code to run copy/build script here
   Write-Host "Done copying...";
}