Property '[...]' does not exist on type 'VueConstructor<Vue>'

[Byloth]

Version

2.6.12

Reproduction link

https://codepen.io/Byloth/pen/mdEzMGX

N.B.: It works, here on CodePen... But it won't work inside a real Vue project.

Steps to reproduce

1. Create a new TypeScript Vue Project using Vue CLI.
2. Copy & Paste this mixin inside main.ts file (right above Vue app initialization and mount).

Vue.mixin({
    data: () => ({ aRandomMixinProperty: "John Doe" }),

    mounted()
    {
        this.aRandomMixinProperty = "John Snow";
    }
});

3. Try to build the project.

What is expected?

It should compile correctly.

What is actually happening?

It doesn't compile at all.

ERROR in /path/to/project/src/main.ts(15,14):
15:14 Property 'aRandomMixinProperty' does not exist on type 'VueConstructor<Vue> | ComponentOptions<Vue, DefaultData<Vue>, DefaultMethods<Vue>, DefaultComputed, PropsDefinition<...>, Record<...>>'.
  Property 'aRandomMixinProperty' does not exist on type 'VueConstructor<Vue>'.
    13 |     mounted()
    14 |     {
  > 15 |         this.aRandomMixinProperty = "John Snow";
       |              ^
    16 |     }
    17 | });
    18 |
Version: typescript 4.0.5
Time: 2856ms

---

As workaround, you could use:

/* main.ts */

Vue.mixin({
    data: () => ({ aRandomMixinProperty: "John Doe" }),

    mounted()
    {
        (this as Vue).aRandomMixinProperty = "John Snow";
    }
});

... and...

/* mixins.d.ts */

declare module "vue/types/vue"
{
    interface Vue
    {
        aRandomMixinProperty: string;
    }
}

[posva]

The workaround you mentioned is the way to go: you need to tell TS that global properties have been added

[Byloth]

Oh... Ok, @posva!

But... Why doesn't the typings work like the extend method?
Why data is ignored?

Aren't the typings rules written correctly, is it by choice or is it simply impossible to define them correctly?
May I contribute to fix this problem? Could it be worth investing some time?

[seupedro]

For those coming from Google like me, check this thread instead #8721.
There are some workarounds like this, this, this, and this.

And for those who likes to take time read and understand, check this reply on same thread.