Debounced and Async Form Validation
Learn how to validate form fields against an async source (like checking username availability) without spamming the server on every keystroke.
Some validation rules can't be checked purely on the client — verifying a username or email isn't already taken requires an API call. Running that check on every keystroke would flood the server with requests for a value the user hasn't even finished typing, so async validation is almost always paired with debouncing: waiting until the user pauses typing before firing the actual validation request.
It's like asking a librarian to confirm whether a book title is available, but you keep revising your request before they've answered your first query — a careful librarian ignores their answer to your earlier, now-outdated question if you've since asked about a different title, rather than confusingly telling you about the wrong book after the fact.