Validate enum prop in vue

It's very common to validate prop as enum:

<script>
const VALID_TYPES = ['primary', 'secondary', 'warning']
const VALID_SIZES = ['mini', 'small', 'semi-wide', 'wide']

export default {
  props: {
    type: {
      type: [String, Array],
      default: 'primary',
      validator: type => VALID_TYPES.includes(type),
    },
    size: { 
      type: String, 
      default: 'small', 
      validator: size => VALID_SIZES.includes(size) 
    },
  },
}
</script>

What happens if default is not provided (and is not required)