I'm trying to validate an object's field using yup.

These are my requirements:

1. The field should either be a string or undefined.


2. If it is undefined, it should be given a default value.


3. If it is a value of some other type, it should not be coerced into a string. Instead, it should be incompatible and should result in an error.

I've tried the following so far:

A. The following schema does not prevent type coercion. e.g. when the value is a number, it passes the validation. However, I want it to fail the validation.

const schema = yup.object().shape({

  myField: yup.string().default('myDefaultString')

});

B. The following schema prevents type coercion, but it fails when I pass an undefined value. I actually want the value to be 'myDefaultString' if an undefined value is given.

const schema = yup.object().shape({

  myField: yup.string().strict(true).default('myDefaultString')

});

C. The following schema has the same result as option B.

const schema = yup.object().shape({

  myField: yup.string().strict(true).notRequired().default('myDefaultString')

});

D. Use strict: true as part of the options when using validateSync method to validate the schema. Again, this has the same result as B.

Would appreciate any help with this, thanks!