Optional validation for hidden fields

// This is the shared logic for whether the mightBeHidden field is visible and required
const shouldShow = (document) => {
  return document.conditionField === 'show'
}

export default {
  type: 'document',
  name: 'myDocumentType',
  title: 'My Document Type',
  fields: [
    {
      // when you enter "show" into this text field,
      // the mightBeHidden field will appear and be required
      type: 'string',
      name: 'conditionField',
      title: 'Condition Field',
    },
    {
      type: 'string',
      name: 'mightBeHidden',
      title: 'Might Be Hidden',
      validation: (rule) =>
        rule.custom((currentValue, {document}) => {
          // in a custom validation rule, check if the field should be shown, and if yes, show an error if the value is not set
          if (shouldShow(document) && currentValue === undefined)
            return "This is required when conditionField === 'show'"
          // if we are not showing the field, or if the field has a value
          // then the validation passes
          return true
        }),
      hidden: ({document}) => {
        // use the shared function to decide whether to show the field
        return !shouldShow(document)
      },
    },
  ]
}