How to mock Vue Mixins during unit testing using vue-test-utils and jest?

There are two ways:

1. You can use createLocalVue, and register a mixin on that localVue class:

const localVue = createLocalVue()
localVue.mixin(myMixin)

const wrapper = shallow(Post, {
    localVue,
})

2. You can pass mixins in the mounting options:

const wrapper = shallow(Post, {
    mixins: [myMixin],
})

For those using Vue 3 with Vue Test Utils, then you just need to mock the individual method, for example with Jest. Pass in your myMixin as normal, and then spy on the method you want to mock:

    const wrapper = mount(Post, {
        global: {
            mixins: [myMixin],
        },
    } as any)

    jest.spyOn(wrapper.vm, 'myMixinMethodToMock').mockImplementation()

Note that Jest mocks it without caring that the method is on the mixin, not the Vue component.

I managed to mock the mixin methods with jest spies like this:

/// MyComponent.spec.js
describe('MyComponent', () => {
  let wrapper
  let localVue
  let store
  let spies = {}
  
  beforeEach(async () => {
    spies.mixinMethodName = jest.spyOn(MyComponent[1].methods, 'spies.mixinMethodName')
    ({ localVue, store } = (... custom factory ...)
    wrapper = await shallowMount(MyComponent, { localVue, store })
  })

  it('check mixin methods calls', () => {
    expect(spies.mixinMethodName).toHaveBeenCalled()
  })
})

Of course the spies object and its attached method can be customized as your wish.

The weakness of this method is that it relies on the order of the mixins entered in the real Vue component. For this example, this looks like:

/// MyComponent.vue
<script>
  export default {
    components: { ...components... },
    mixins: [mixin1, mixin2ToBeTested],
    data () {}
    ....
}
</script>