Number but allow empty string

I have a legacy project where a field can be a number, but the absence of value is defined by an empty string (instead of null).

I am looking for something like nullable (yup.number().allowEmptyString()) or yup.mixed.oneOf(['', yup.number()]

Does anyone have an idea on how to handle this in the cleanest way?

Currently I am doing

const schema = yup
  .mixed()
  .test(fn) // value === '' || yup.number().isValidSync(value)
  .transform(fn) // value === '' ? value : +value

but this loses beautiful syntax of Yup and just feels dirty since yup.number() is burried inside test, but may be further customized for other fields.

Thanks

[MarkMurphy]

I’m a little confused as to what the recommended solution is here.

[NicolasLetellier]

I had a bit of hard times finding the right combination for my case (I don’t know why but I wasn’t able to make other solutions working in my code), so here is my working validation schema if it can help anybody:

// helper for yup transform function
function emptyStringToNull(value, originalValue) {
  if (typeof originalValue === 'string' && originalValue === '') {
    return null;
  }
  return value;
}

// to validate a number with min and max and allowing empty string
const mySchema = yup.number().min(-0.2).max(0.2).transform(emptyStringToNull).nullable();