This example shows how you can use Ajax to do server-side form data validation without a page reload.
In the form below enter a user id. By default the user ids "greg", "duke" and "ねこ" are taken. If you attempt to enter a user id that has been taken an error message will be displayed next to the form field and the "Create Account" button will be disabled. After entering a valid user id and selecting the "Create Account" button that user id will be added to the list of user ids that are taken.