Instructor: let's add some submit validation to our form. When the user hits submit, we're going to check if their user name already exists and return an appropriate error message. To do this, we firstly need to import submission error from Redux Form inside our app component, which is used to differentiate IO errors from validation errors.
Inside our submit method, which is being fired on submit, we're going to add an if statement. The alert that we have down here is going to come in the else section. Here's where we're going to check to see if the user name is available. In the real world, we would query our database, but for the sake of this example, we'll mark some already-taken user names.
If our user name is one of those, then we want to throw a new submission error, passing in an object, and the object keys must pertain to form field names, in this case, user name, and the value can be an appropriate error message. Let's save and refresh. We'll add some example values.
I've used a user name which is already taken. I'll hit submit. Here's the appropriate error message...