Testing Vuejs and TypeScript with Jest causes an error "Property 'xxx' does not exist on type 'CombinedVueInstance'

Question

I'm trying to create a basic test for a function in Vue component and failing miserably.

main.ts

new Vue({
  render: (h) => h(App),
  router,
  store,
}).$mount('#app')

App.ts

const App = Vue.extend({
    components: { MainContainer },
    data() {
        return {
            msg: 'Test',
        }
    },
    name: 'App',
    template: `<MainContainer />`,
})

export default App

MainContainer.vue

const MainContainer = Vue.extend({
    components: {
        Content,
    },
    methods: {
        sum: (a: number, b: number): number => a + b,
    },
})

export default MainContainer

MainContainer.spec.ts

import { createLocalVue, shallowMount } from '@vue/test-utils'

describe('MainComponent.vue', () => {
    const localVue = createLocalVue()

    test('sum method should add number together', () => {
    const wrapper = shallowMount(MainContainer, { localVue })
    expect(wrapper.vm.sum(1, 2)).toEqual(3)
})

Tests fails with the error

Property 'sum' does not exist on type 'CombinedVueInstance<Vue, object, object, object, Record<never, any>>'.

The same behavior I see if I try to test data.msg from App.ts. I'd appreciate any help. Thanks

Answer 1

You must specify type of wrapper.vm

A solution that works is to specify type to any :

expect((wrapper.vm as any).sum(1, 2)).toEqual(3)