When you need to validate a form field against information stored on a server, such as checking if a username is already taken, you must perform asynchronous validation. This means your JavaScript code will send a request to a server and wait for a response before deciding if the input is valid. Unlike synchronous validation, which runs instantly and only uses local logic, asynchronous validation involves network calls that can take some time to complete. This is common in scenarios like verifying the availability of an email address or username during registration.

Asynchronous validation introduces unique user experience (UX) considerations. Since users must wait for a server response, you should always provide clear feedback, such as displaying a loading message like "Checking availability..." while the request is in progress. If the check fails due to a network error, let users know and encourage them to try again. Avoid blocking the entire form while waiting for a single field to validate; instead, disable only the relevant field or show a spinner next to it. Good UX keeps users informed and prevents confusion or frustration during the validation process.