When building forms in React, you often encounter situations where validation cannot be handled solely on the client side. Some rules require checking with a backend or external service. For example, you may need to verify if a username is available or confirm that an email address is not already registered. These scenarios require asynchronous validation because they depend on information not immediately available in the browser. Zod, while primarily a synchronous validation library, provides mechanisms to support async validation needs by allowing you to run asynchronous checks after the initial schema validation step.

The main challenge with async validation is that you must wait for a response from an external source, such as a server, before you can determine if the input is valid. This introduces complexity in your form logic, as you need to manage loading states, error reporting, and possibly debounce rapid user input to avoid excessive network requests. Zod's approach is to let you run your standard synchronous validation first, then perform any async checks as a separate step, often in your form submission handler or within React hooks.

To integrate async validation with React forms and Zod, you typically follow a two-step process. First, define your Zod schema to handle all rules that can be checked synchronously—such as required fields, string length, or pattern matching. Next, after passing this initial validation, perform any necessary asynchronous checks, such as querying your backend to see if a username is already taken. In React, you can handle this by using state to track the async validation status, show loading indicators, and display errors returned from the server. You might use a custom hook or add logic to your form's onSubmit handler to ensure async validation is handled cleanly and user feedback is prompt and clear.