can't spy on nested function call using Vitest and Pinia

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05-10-2024

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Spy on Nested Function Calls with Vitest and Pinia: A Guide to Solving the Mystery

Testing deeply nested function calls can be tricky, especially when using libraries like Vitest and Pinia. Imagine this scenario: you have a component that relies on a Pinia store, which in turn calls a nested function that you want to test. You might try spying on the nested function, but find that your tests aren't working as expected. This article will explore why this happens and provide solutions to effectively spy on nested function calls in your Vitest and Pinia tests.

The Scenario: A Case of the Missing Spy

Let's say we have a simple component that interacts with a Pinia store:

// src/components/MyComponent.vue
<template>
  <button @click="handleClick">Click Me</button>
</template>

<script setup>
import { useMyStore } from '../stores/myStore';

const store = useMyStore();

const handleClick = () => {
  store.doSomething();
};
</script>

Our Pinia store might look like this:

// src/stores/myStore.js
import { defineStore } from 'pinia';

export const useMyStore = defineStore('myStore', {
  state: () => ({
    count: 0,
  }),
  actions: {
    doSomething() {
      const nestedFunction = () => {
        this.count++;
      };
      nestedFunction();
    },
  },
});

Now, let's write a Vitest test to spy on the nestedFunction:

// src/tests/MyComponent.spec.js
import { mount } from '@vue/test-utils';
import MyComponent from '../components/MyComponent.vue';
import { createPinia } from 'pinia';

describe('MyComponent', () => {
  it('should call nestedFunction when button is clicked', () => {
    const wrapper = mount(MyComponent, {
      global: {
        plugins: [createPinia()],
      },
    });
    const nestedFunction = jest.fn(); // Trying to spy on the nested function
    wrapper.vm.store.doSomething = jest.fn(() => {
      nestedFunction(); 
    });

    wrapper.find('button').trigger('click');

    expect(nestedFunction).toHaveBeenCalled();
  });
});

The Problem: Why the Spy Fails

Running this test will likely fail. The nestedFunction spy will not have been called. Why? Because the nestedFunction is defined within the doSomething action and is only accessible within the scope of that action. Our test, which attempts to spy on the function outside this scope, fails to catch it.

The Solution: Leveraging Mocking and Spies

The key to addressing this lies in understanding the context of the nestedFunction. We need to mock the doSomething action and ensure the nestedFunction within it is correctly spied upon.

Here's an updated test demonstrating this approach:

// src/tests/MyComponent.spec.js
import { mount } from '@vue/test-utils';
import MyComponent from '../components/MyComponent.vue';
import { createPinia, setActivePinia } from 'pinia';
import { useMyStore } from '../stores/myStore';

describe('MyComponent', () => {
  it('should call nestedFunction when button is clicked', () => {
    setActivePinia(createPinia());
    const store = useMyStore();

    const nestedFunction = jest.fn();

    // Mock the doSomething action, ensuring the spy is in scope
    store.doSomething = jest.fn(() => {
      nestedFunction();
    });

    const wrapper = mount(MyComponent);

    wrapper.find('button').trigger('click');

    expect(nestedFunction).toHaveBeenCalled();
  });
});

In this improved test:

1. We directly access the useMyStore and mock the doSomething action.
2. The nestedFunction spy is now defined within the mocked doSomething function, ensuring it is in the correct scope.

Important Note: The setActivePinia function is crucial to ensure that the mocked store is used in the mounted component.

Beyond the Basics: Additional Considerations

• Isolation: To ensure thorough testing, consider using a mocking library like vitest-mock to mock dependencies within your doSomething action. This helps isolate your component's behavior and prevent unintended interactions with other parts of your application.
• State Management: If your doSomething action manipulates store state, remember to verify the state changes within your test.

Conclusion

Testing nested function calls within a Pinia store is possible with Vitest. The key is to understand the scope of your nested functions and ensure that your spies are defined in the correct context. By effectively mocking and spying on functions within your Pinia actions, you can create robust and comprehensive tests for your Vue applications.