Props typing in Vue.js 3 with TypeScript

Question

I'm trying to type hint my props in a Vue 3 component, with composition API.

So, I'm doing this:

<script lang="ts">
import FlashInterface from '@/interfaces/FlashInterface';
import { ref } from 'vue';
import { useStore } from 'vuex';

export default {
    props: {
        message: {
            type: FlashInterface,
            required: true
        }
    },
    setup(props): Record<string, unknown> {
        // Stuff
    }
};

My FlashInterface looks like this:

export default interface FlashInterface {
    level: string,
    message: string,
    id?: string
}

This interface works well except in this case where I got this error:

ERROR in src/components/Flash.vue:20:10
TS2693: 'FlashInterface' only refers to a type, but is being used as a value here.
    18 |    props: {
    19 |        message: {
  > 20 |            type: FlashInterface,
       |                  ^^^^^^^^^^^^^^
    21 |            required: true
    22 |        }
    23 |    },

I don't understand why TypeScript thinks this is a value.

What am I missing?

Answer 1

You should use it with PropType imported from vue like Object as PropType<FlashInterface>:

import FlashInterface from '@/interfaces/FlashInterface';
import { ref,PropType, defineComponent } from 'vue';
import { useStore } from 'vuex';

export default defineComponent({
    props: {
        message: {
            type: Object as PropType<FlashInterface>,
            required: true
        }
    },
    setup(props) {
            // Stuff
        }
});

Note : you should create your component using defineComponent in order to get the types inference.

script setup

You could define the props using defineProps function in two ways :

<script setup lang="ts">
import FlashInterface from '@/interfaces/FlashInterface';
import { ref,PropType, defineComponent , defineProps} from 'vue';
interface IProps{
   message : FlashInterface
}
const props = defineProps<IProps>()

or

<script setup lang="ts">
...
const props = defineProps({
        message: {
            type: Object as PropType<FlashInterface>,
            required: true
        }
    })

Answer 2

The compiler is complaining about missing a reference for a (custom) constructor during type checking (links to legacy docs but works the same way with the most recent version of Vue).

In Typescript, you might want to think of interfaces as a contract that an entity should conform to, so they aren't really a constructor, and therefore, we need to provide an implementation of those interfaces.

Since you are on Typescript, if you need to keep the interface, consider using the class equivalent:

// Name the constructor whatever you like,
// but I would personally prefix interfaces with an "I"
// to distinguish them with the constructors

class Flash implements FlashInterface {
  level: string;
  message: string;
  id?: string

  constructor() {
    // Be sure to initialize the values for non-nullable props
    this.level = '';
    this.message = '';
  }
}

export default {
  name: 'Home',

  props: {
    message: Flash
  }
}

An excerpt from the doc:

In addition, type can also be a custom constructor function and the assertion will be made with an instanceof check. For example, given the following constructor function exists:

props: {
  message: {
    type: function Person(firstName, lastName) {
      this.firstName = firstName
      this.lastName = lastName
    }
  }
}

And of course, another alternative would be as suggested in the other post with PropType. Either one will do. It's just a matter of preference, I guess.