How to create conditional assignment in initialValues of Formik

I have initialValues in Formik but I want to assign values in accordance with another field.

I have this code:

const initialValues: MyValues = {
    fieldType: fieldType.FIELDA,
    fieldInformation: '',
    fieldB: '', 
    dateRange: {
      from: lastWeek,
      to: today,
    },
  };

I want something like:

const initialValues: MyValues = {
    fieldType: fieldType.FIELDA,
    fieldInformation: '',
    fieldB: fieldType === fieldType.FieldA ? 'all' : '', 
    dateRange: {
      from: lastWeek,
      to: today,
    },
  };

So fieldB depends on the value of fieldType. How can I achieve that? The problem is that fieldType is a radioButton and I want to initialize with ” if fieldType.FIELDB is chosen.