TypeScript support for Vue3 Composition components unit testing

When testing Vue 3 Composition API-based components in TypeScript, you may encounter a TypeScript error while testing your components.

For example, let's consider a simple component that displays the value from an input field:

<script setup lang="ts">
import { ref } from 'vue';

const msg = ref('Hello World!');
</script>

<template>
  <h1 id="msgDisplay">{{ msg }}</h1>
  <input v-model="msg" id="msgInput" />
</template>

Suppose you want to write a unit test for this component:

it('Displays message correctly', async () => {
  const wrapper = mount(DummyComponent, {});
  const inputField = wrapper.find('#msgInput');
  await inputField.setValue('test');
  const msgDisplay = wrapper.find('#msgDisplay');
  expect(msgDisplay.html()).toContain('test');
  // TypeScript error
  expect(wrapper.getComponent(DummyComponent).vm.msg).toBe('test');
});

During the test, you might encounter the TypeScript error: Vue: Property msg does not exist on type. To resolve this error, two small modifications are needed.

Firstly, expose the msg variable within the component:

defineExpose({
  msg,
});

Finally, within the test, access the component using the wrapper.getComponent method:

expect(wrapper.getComponent(DummyComponent).vm.msg).toBe('test');

Voilà! 🚀 With these changes, the TypeScript error should be resolved.