Why does a non-required prop does not accept null?

[LeBenLeBen]

I use a component from a library that has the following props definition:

defineComponent({
  props: {
    modelValue: {
      type: [String, Number],
      default: null
    }
  }
}

When I pass null as a value for modelValue, it works fine with Vue (no props warning), but vue-tsc does not allow it and throws the following error:

(property) modelValue?: string | number | undefined
Type 'null' is not assignable to type 'string | number | undefined'.ts(2322)

I’m wondering why it allows undefined but not null? This is different from Vue internal props validator. Having to explicitly accept null for every optional props sounds weird to me, but I’m probably missing something?

[johnsoncodehk]

This is defineComponent type behavior, you can try type: [String, Number] as PropType<string | number | null> instead of.