Mocking composables and stores in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when mocking a composable's return value?

Given a Vue component that uses a composable returning a reactive count, and the composable is mocked to return a fixed count, what will the component render?

Vue

/* Composable: useCounter.js */
import { ref } from 'vue';
export function useCounter() {
  const count = ref(0);
  function increment() { count.value++; }
  return { count, increment };
}

/* Component: CounterDisplay.vue */
<template>
  <div>{{ count }}</div>
</template>
<script setup>
import { useCounter } from './useCounter';
const { count } = useCounter();
</script>

/* Test file snippet */
vi.mock('./useCounter', () => ({
  useCounter: () => ({ count: 42 })
}));

A The component throws a runtime error due to missing ref wrapper.

B The component renders '0' because the mock does not update reactivity.

C The component renders '42' as the count value.

D The component renders 'undefined' because count is not reactive.

💡 Hint

Think about what the mock returns and how the component uses it.

The mock replaces the composable's return with a fixed value. Since the component uses the mocked count directly, it renders '42'.

🔧 Debug

intermediate

Why does mocking a Pinia store's state fail with a TypeError?

In a test, mocking a Pinia store's state directly causes a TypeError: Cannot read property 'value' of undefined. What is the likely cause?

Vue

import { setActivePinia, createPinia } from 'pinia';
import { useUserStore } from '@/stores/user';

setActivePinia(createPinia());

vi.mock('@/stores/user', () => ({
  useUserStore: () => ({
    state: { name: 'Alice' }
  })
}));

const store = useUserStore();
console.log(store.name);

A The mock returns a plain object instead of a reactive store, so accessing reactive properties fails.

B Pinia stores cannot be mocked at all, causing all mocks to fail.

C The test environment lacks a proper Vue instance, causing reactivity errors.

D The store is not registered in Pinia, so useUserStore returns undefined.

💡 Hint

Consider how Pinia stores expose state and reactivity.

Pinia stores return reactive objects. Mocking with plain objects loses reactivity, causing errors when code expects refs or reactive proxies.

📝 Syntax

advanced

Which mock setup correctly mocks a composable returning a reactive ref?

Choose the correct way to mock a composable that returns a reactive ref count initialized to 10.

Vue

/* Original composable: useCount.js */
import { ref } from 'vue';
export function useCount() {
  const count = ref(0);
  return { count };
}

A vi.mock('./useCount', () => ({ useCount: () => ({ count: 10 }) }));

B vi.mock('./useCount', () => ({ useCount: () => ({ count: computed(() => 10) }) }));

C vi.mock('./useCount', () => ({ useCount: () => ({ count: reactive(10) }) }));

D vi.mock('./useCount', () => ({ useCount: () => ({ count: ref(10) }) }));

💡 Hint

Remember how the original composable returns a ref.

The original composable returns a ref. The mock must also return a ref to keep the same interface and reactivity.

❓ state_output

advanced

What is the value of store.count after mocking and incrementing?

Given a Pinia store with a count state and an increment action, if the store is mocked to start with count = 5, what is the value of count after calling increment() once?

Vue

/* Store definition */
import { defineStore } from 'pinia';
export const useCounterStore = defineStore('counter', {
  state: () => ({ count: 0 }),
  actions: {
    increment() { this.count++; }
  }
});

/* Test mock */
vi.mock('@/stores/counter', () => ({
  useCounterStore: () => ({
    count: 5,
    increment() { this.count++; }
  })
}));

const store = useCounterStore();
store.increment();
const result = store.count;

A 5

B 6

C NaN

D undefined

💡 Hint

Check how the mocked increment method modifies the count property.

The mocked store starts with count 5. Calling increment increases count by 1, so the final value is 6.

🧠 Conceptual

expert

Why is it important to mock composables with the same reactive structure as originals?

When mocking Vue composables or Pinia stores, why must the mock preserve the original's reactive structure (e.g., refs, reactive objects) instead of returning plain values?

A Because Vue components rely on reactivity to update the UI, and mocks without reactive wrappers break this, causing stale or missing updates.

B Because plain values cause syntax errors in Vue templates when used inside components.

C Because testing frameworks require all mocks to be reactive to run tests successfully.

D Because only reactive objects can be imported in Vue components without errors.

💡 Hint

Think about how Vue tracks changes to update the screen.

Vue uses reactivity to detect changes and update the UI. If mocks return plain values, Vue cannot track changes, so the UI won't update as expected during tests.