Vue2 with typescript, Property does not exist on type

Question

I have the below Vue component:

<template>
    <v-snackbar bottom :color="color" v-model="snackbar">
        {{ msg }}
        <v-btn flat @click.native="close()">Close</v-btn>
    </v-snackbar>
</template>

<script lang="ts">
    import Vue from 'vue';
    import VueEvent from '../../VueEvent';

    export default Vue.extend({
        data(): object {
            return {
                snackbar: false,
                msg: '',
                color: '',
            };
        },
        created() {
            VueEvent.listen('snackbar', (data) => {
                this.snackbar = true;
                this.msg = data.msg;
                this.color = data.color;
            });
            this.malert();
        },
        methods: {
            close(): void {
                this.snackbar = false;
            }
        }
    });
</script>

<style scoped>

</style>

When Typescript compiles I get the following error:

Property 'snackbar' does not exist on type 'CombinedVueInstance<Vue, object, { close(): void; }, {}, Readonly<Record<never, any>>>'.
    28 |         methods: {
    29 |             close(): void {
  > 30 |                 this.snackbar = false;
       |                      ^
    31 |             }
    32 |         }
    33 |     });

Does anyone know how I can solve this problem, or explain why its happening?

Answer 1

Well, I don't have a good answer for you, but I have theory:

The original type declaration resides in vue/types/options.d.ts:

type DefaultData<V> =  object | ((this: V) => object); // here is the kicker
Data=DefaultData<V>
data?: Data;

And I found that:

data():object { ... // this.snackbar does not exist
data(){ ... // this.snackbar does exist.
data(): any { ... // this.snackbar does exist.
data(): { snackbar: boolean; msg: string; color: string } { ... // Also valid

I think without your object declaration typescript will think that data is DefaultData<V> = object. But once you say it returns an object, data will suddenly match ((this: V) => object) instead. Now typescript expects this to be of type V (which I assume is a vue-instance) and since that vue instance does not have a snackbar in it's definition, boom, typescript throws.

Lot's of guessing here, but I think pretty much anything except explicitly returning object would work to not match that second signature in DefaultData<V>.