How can I implement custom validation in React Final Form?

How can I implement custom validation in React Final Form?

You can implement custom validation in React Final Form by using the validate prop on your field components. This allows you to define a function that checks the field's value and returns an error message if the validation fails.

Can you show me an example of a custom validation function?

Sure! Here's a simple example:

const validateEmail = value => {
  if (!value) {
    return 'Required';
  } else if (!/\S+@\S+\.\S+/.test(value)) {
    return 'Invalid email address';
  }
};

You can use this function in your field like this:

<Field name="email" validate={validateEmail} />

What if I want to validate multiple fields at once?

You can use the validate prop on the form level to validate multiple fields. Just create a validation function that checks all the fields and returns an object with error messages for each field.

Thanks! Is there a way to show error messages conditionally?

Yes, you can conditionally render error messages based on the validation results. Use the meta object provided by the field to check for errors and display them accordingly.