til/javascript/custom-type-checking-error-messages-with-yup.md

Custom Type Checking Error Messages With Yup

In Yup Schemas Are Validated Asynchronously, I showed how to create a simple schema that allows you to enforce that a value is a number.

const numSchema = yup.number();

If we use this schema to validate something that isn't a number, Yup will provide a lengthy default message. Here is what we get if we validate against 'hey':

this must be a number type, but the final value was: NaN (cast from the value "hey").

This value isn't necessarily suitable for displaying to a user. We can customize the type checking error message by redefining our schema with the typeError() function:

const numSchema = yup.number().typeError("Invalid number");