Mocking methods on a Vue instance during TDD

Question

I\'m learning TDD whilst building my Vue app, and trying to abide by the strict laws of only writing enough production code to satisfy a failing unit test. I am really enjoy

Answer 1

Solution 1: jest.spyOn(Component.methods, 'METHOD_NAME')

You could use jest.spyOn to mock the component method before mounting:

import MyComponent from '@/components/MyComponent.vue'

describe('MyComponent', () => {
  it('click does something', async () => {
    const mockMethod = jest.spyOn(MyComponent.methods, 'doSomething')
    await shallowMount(MyComponent).find('button').trigger('click')
    expect(mockMethod).toHaveBeenCalled()
  })
})

Solution 2: Move methods into separate file that could be mocked

The official recommendation is to "abstract the hard parts away", and use Jest's various mocking mechanisms to mock the abstracted module invoked by the component under test.

For example, to verify a click-handler is invoked:

1. Move the click-handler's body into a shared JavaScript file.
2. Import the shared module into the component under test and in your tests (make sure to use the same import path in both cases).
3. Call jest.mock() to mock the exported functions of the shared module.
4. Reset the mock in your test suite's beforeEach(). This might only be necessary when there are multiple tests in the suite.

// @/components/MyComponent/utils.js
export function doSomething() { /*...*/ } //1️⃣

// @/components/MyComponent/MyComponent.vue (<script>)
import { doSomething } from '@/components/MyComponent/utils' //2️⃣

export default {
  methods: {
    onClick() {
      doSomething() //1️⃣
    }
  }
}

// @/test/MyComponent.spec.js
import { doSomething } from '@/components/MyComponent/utils' //2️⃣
jest.mock('@/components/MyComponent/utils') //3️⃣

describe('MyComponent', () => {
  beforeEach(() => doSomething.mockClear()) //4️⃣

  it('click does something', async () => {
    await shallowMount(MyComponent).find('button').trigger('click')
    expect(doSomething).toHaveBeenCalled()
  })
})

Solution 3: setMethods() (pre v1.0)

Use setMethods() (deprecated as of v1.0) to overwrite a component method:

describe('MyComponent', () => {
  it('click does something', async () => {
    // Option A:
    const mockMethod = jest.fn()
    const wrapper = shallowMount(MyComponent)
    wrapper.setMethods({ doSomething: mockMethod })
    await wrapper.find('button').trigger('click')
    expect(mockMethod).toHaveBeenCalled()

    // Option B:
    const mockMethod = jest.fn()
    const wrapper = shallowMount(MyComponent, {
      methods: {
        doSomething: mockMethod
      }
    })
    await wrapper.find('button').trigger('click')
    expect(mockMethod).toHaveBeenCalled()
  })
})

demo