Vue how to test component with slot and slot-props

Question

I want to test this FooComponent:

<div>
  <slot :fn="internalFn" />
</div>

It's used like that (e.g. in ParentComponent):

<FooComponent>
  <template slot-scope="slotProps">
    <BarComponent @some-event="slotProps.fn" />
  </template>
</FooComponent>

So I want to test how my component reacts on calling this "fn" from slot props. The easiest way I see is to take method itself and call it, like that:

cosnt wrapper = shallowMount(FooComponent, /* ... */)
wrapper.vm.methods.internalFn(/* test payload */)
expect(wrapper.emitted()).toBe(/* some expectation */)

But this is well known as anti-pattern about testing internal implementation. So instead I would like to test it via prop fn passed into slot, because it's also some sort of component interface, like component own props.

But how to to test props passed in slot? I can imagine it's working only in case if I test that ParentComponent something like that:

const wrapper = shallowMount(ParentComponent, /* ... */)
const foo = wrapper.find(FooComponent)
wrapper.find(BarComponent).vm.$emit('some-event', /*...*/)
/* write expectations against foo */

But that feels like tests for FooComponent inside tests for ParentComponent

Maybe there is a better way to do it?

Answer 1

UPD: below the line is the original answer that I gave years ago. For today I would say that one of these approaches would be good enough:

1. Don't test the slot method, but test the user-facing feature that relies on it. Try to test it manually first, notice how do you do it, then try to write a similar test. E.g. with testing-library/vue

2. If the first option is too hard to do, try to come up with some fake testing component. The idea is very similar to what I described in the question

But that feels like tests for FooComponent inside tests for ParentComponent

But instead of using the ParentComponent just create some very simple inline component (right in your FooComponent.spec.js file), that uses the component with a slot.

---

Original answer that was given in 2020:

Since there are no answers so I just share what I ended up with.

I decided to test the internal method. It's not cool, but at least, because I use typescript, I have a type-safe test, which will fail with a clear type error if I rename or modify the method which I test. Looks like this:

import Foo from '@/components/foo/Foo.ts'
import FooComponent from '@/components/foo/Foo.vue'

/*...*/

cosnt wrapper = <Foo>shallowMount(FooComponent, /* ... */)

// notice that because I passed `Foo` which is a class-component, 
// I have autocomplete and type checks for vm.*
wrapper.vm.internalFn(/* test payload */)

expect(wrapper.emitted()).toBe(/* some expectation */)