Testing With Jest | Advanced Vuejs

Unit Testing with Jest

Jest is a popular testing framework for JavaScript, providing a comprehensive and easy-to-use toolset for unit testing. This guide explores how to use Jest for testing VueJS applications.

Key Points:

Jest provides a powerful testing framework with built-in assertion libraries, mocking, and coverage reporting.
Vue Test Utils is a utility library that makes it easier to test Vue components with Jest.
Unit testing ensures that individual components and functions work as expected, improving the reliability of your application.

Setting Up Jest

To get started with Jest in a VueJS project, install the necessary dependencies:

# Install Jest and Vue Test Utils
npm install --save-dev jest @vue/test-utils vue-jest babel-jest

# Create a Jest configuration file
echo "module.exports = {
  preset: '@vue/cli-plugin-unit-jest/presets/default'
}" > jest.config.js

Writing Your First Test

Here's an example of a simple test for a Vue component using Jest and Vue Test Utils:

// src/components/HelloWorld.vue




// tests/unit/HelloWorld.spec.js
import { shallowMount } from '@vue/test-utils'
import HelloWorld from '@/components/HelloWorld.vue'

describe('HelloWorld.vue', () => {
  it('renders props.msg when passed', () => {
    const msg = 'new message'
    const wrapper = shallowMount(HelloWorld, {
      props: { msg }
    })
    expect(wrapper.text()).toMatch(msg)
  })
})

Running Tests

To run your Jest tests, use the following command:

# Run Jest tests
npm run test:unit

This command runs all tests in your project and provides a summary of the test results.

Mocking Dependencies

Jest provides powerful mocking capabilities, allowing you to mock dependencies and functions. Here's an example of how to mock an API call:

// src/api.js
export const fetchData = () => {
  return Promise.resolve('data')
}

// src/components/DataComponent.vue




// tests/unit/DataComponent.spec.js
import { shallowMount } from '@vue/test-utils'
import DataComponent from '@/components/DataComponent.vue'
import * as api from '@/api'

jest.mock('@/api')

describe('DataComponent.vue', () => {
  it('fetches data on creation', async () => {
    api.fetchData.mockResolvedValue('mocked data')
    const wrapper = shallowMount(DataComponent)
    await wrapper.vm.$nextTick()
    expect(wrapper.text()).toMatch('mocked data')
  })
})

Testing Vuex

You can also test Vuex stores using Jest. Here's an example of a test for a Vuex store module:

// src/store/modules/counter.js
export const state = () => ({
  count: 0
})

export const mutations = {
  increment(state) {
    state.count++
  }
}

// tests/unit/counter.spec.js
import { mutations } from '@/store/modules/counter'

describe('mutations', () => {
  it('increment', () => {
    const state = { count: 0 }
    mutations.increment(state)
    expect(state.count).toBe(1)
  })
})

Snapshot Testing

Jest also supports snapshot testing, which allows you to capture the rendered output of a component and compare it to a reference snapshot:

// tests/unit/HelloWorld.spec.js
import { shallowMount } from '@vue/test-utils'
import HelloWorld from '@/components/HelloWorld.vue'

describe('HelloWorld.vue', () => {
  it('matches snapshot', () => {
    const msg = 'new message'
    const wrapper = shallowMount(HelloWorld, {
      props: { msg }
    })
    expect(wrapper.html()).toMatchSnapshot()
  })
})

Best Practices

Follow these best practices when writing tests with Jest:

Write Clear and Descriptive Tests: Ensure your test descriptions clearly state what is being tested.
Keep Tests Small and Focused: Write tests that focus on a single unit of functionality.
Use Mocks and Spies: Use mocks and spies to isolate the unit being tested and avoid testing dependencies.
Leverage Snapshot Testing: Use snapshot testing to verify the rendered output of your components.
Run Tests Frequently: Run your tests frequently to catch issues early in the development process.

Summary

This guide provided an overview of unit testing with Jest in VueJS applications, including setting up Jest, writing and running tests, mocking dependencies, testing Vuex, and using snapshot testing. By leveraging Jest, you can ensure the reliability and quality of your VueJS applications through comprehensive unit testing.

VueJS - Testing with Jest