>Imagine a username input that should be valid only if the username is not taken
Maybe that is user friendly but for sure I don't like to see the backend bombarded with API calls each time an user types a letter.
Maybe that is user friendly but for sure I don't like to see the backend bombarded with API calls each time an user types a letter.
Huge, huge, massive “no no”.
Likewise you still have to do sever side validation as any client side code can be modified, or you can just send payloads directly to the server. IMHO client side form validation is dangerous as it gives a false sense of security.
The total amount of traffic in both direction is pretty small, and the logic is simple, especially compared to lots of other things your server is typically doing.
You'd probably have to wait until the user moves on to the next field, if there's more, but that's also a little silly, as you'd force the user to go back to a previous field.