Validating User Input

A form can reject a correct email address because of one space at its end. A simple pattern check rejects me@site.com with the space and accepts the same address once the space is removed. Checking the data that a user enters before your app uses it is called validation, and most of the work is deciding what to reject and how to tell the user.

Checking That a Field Is Not Empty

The simplest rule says that a required field must contain text. The sign-up form applies it when the user presses Submit, and it stores the message in state so that the screen can display it.