Composition Api Testing | Advanced Vuejs

Testing Components Using the Composition API

Testing is an essential part of building robust applications. This guide covers how to test components that use the Vue 3 Composition API, ensuring your components work as expected in different scenarios.

Key Points:

Testing components ensures they behave as expected and helps prevent bugs.
The Vue Test Utils library provides utilities for testing Vue components.
Jest is a popular testing framework that works well with Vue Test Utils.

Setting Up the Testing Environment

To get started with testing Vue components, you need to set up the testing environment with Vue Test Utils and Jest:

npm install @vue/test-utils jest jest-transform-stub vue-jest babel-jest @babel/preset-env --save-dev

// jest.config.js
module.exports = {
  preset: '@vue/cli-plugin-unit-jest',
  transform: {
    '^.+\\.vue$': 'vue-jest'
  }
};

// babel.config.js
module.exports = {
  presets: ['@babel/preset-env']
};

Writing a Simple Test

Here is an example of a simple test for a component using the Composition API:


// MyComponent.vue




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

describe('MyComponent.vue', () => {
  it('renders the correct message', () => {
    const wrapper = mount(MyComponent);
    expect(wrapper.text()).toContain('Hello, Vue 3!');
  });

  it('updates the message when the button is clicked', async () => {
    const wrapper = mount(MyComponent);
    await wrapper.find('button').trigger('click');
    expect(wrapper.text()).toContain('Hello, Composition API!');
  });
});

Testing Asynchronous Code

Testing components that involve asynchronous code, such as API calls, requires handling promises and using async/await:


// AsyncComponent.vue




// AsyncComponent.spec.js
import { mount } from '@vue/test-utils';
import AsyncComponent from '@/components/AsyncComponent.vue';

describe('AsyncComponent.vue', () => {
  it('renders loading state initially', () => {
    const wrapper = mount(AsyncComponent);
    expect(wrapper.text()).toContain('Loading...');
  });

  it('renders data when API call is successful', async () => {
    global.fetch = jest.fn(() =>
      Promise.resolve({
        ok: true,
        json: () => Promise.resolve('some data')
      })
    );

    const wrapper = mount(AsyncComponent);
    await flushPromises();
    expect(wrapper.text()).toContain('Data: some data');
  });

  it('renders error message when API call fails', async () => {
    global.fetch = jest.fn(() =>
      Promise.resolve({
        ok: false
      })
    );

    const wrapper = mount(AsyncComponent);
    await flushPromises();
    expect(wrapper.text()).toContain('Network response was not ok');
  });
});

// flushPromises.js
export default function flushPromises() {
  return new Promise(resolve => setImmediate(resolve));
}

Testing Custom Hooks

Custom hooks can also be tested independently by creating a test component that uses the hook:


// useCounter.js
import { ref } from 'vue';

export function useCounter() {
  const count = ref(0);
  const increment = () => {
    count.value++;
  };
  const decrement = () => {
    count.value--;
  };

  return {
    count,
    increment,
    decrement
  };
}

// useCounter.spec.js
import { mount } from '@vue/test-utils';
import { useCounter } from '@/composables/useCounter';

const TestComponent = {
  template: '',
  setup() {
    return useCounter();
  }
};

describe('useCounter', () => {
  it('increments and decrements the count', async () => {
    const wrapper = mount(TestComponent);
    const { count, increment, decrement } = wrapper.vm;

    expect(count.value).toBe(0);

    increment();
    expect(count.value).toBe(1);

    decrement();
    expect(count.value).toBe(0);
  });
});

Best Practices

Follow these best practices when testing components using the Composition API:

Write Tests for All Logic: Ensure all component logic is covered by tests, including computed properties, watchers, and methods.
Use Mocking for External Dependencies: Mock external dependencies such as API calls to ensure tests run reliably and quickly.
Test Custom Hooks Independently: Create test components to test custom hooks independently from the components that use them.
Use async/await for Asynchronous Tests: Use async/await to handle asynchronous code in tests, ensuring they are easy to read and maintain.
Keep Tests Simple and Focused: Write simple and focused tests that cover one piece of functionality at a time.

Summary

This guide provided an overview of testing components using the Vue 3 Composition API. By leveraging these techniques and best practices, you can ensure your Vue 3 applications are robust, maintainable, and free of bugs.

VueJS - Vue 3 Composition API: Testing