How to create yup validator with array of string type for checkboxes

[arpitdalal]

I'm using withYup and yup and I've this as my validator:

const validator = withYup(
  yup.object({
    checkboxes: yup.array(yup.string()),
  }),
);

but when I submit my checkboxes using this validator, I face a weird behavior/bug. If only 1 checkbox is submitted, the validator.validate actually returns a string type instead of an array for some reason but when submitting multiple checkboxes (2 or more), validator.validate gives the correct array type.
NOTE: all the checkboxes have the same name

Please see this example on stackblitz and github repo

Is my validator wrong for this use case or is this a bug? Because I believe, validator.validate should give me the same object as the original formData

[arpitdalal]

A little more digging tells me that there is something wrong in flatten.ts or MultiValueMap.ts which is flattening the one value array

[arpitdalal]

New issue created
Edit: because I believe this is a bug in the lib itself

[airjp73]

I responded on the issue, but responding here as well for completeness. Unfortunately, this behavior is a known and documented limitation. To support this use case, you need to write a validator that can handle it being either a single string or an array of strings. I believe there's a way to preprocess values in yup that you could use to do this, but I don't know how that works off the top of my head.