How to Unit Test a Method in a Vue.js Component using jest

Question

I'm trying to unit test a component method. The question here does not lay out how to access the component method from a unit test.

Specifically, given my Vue component below, how do I access doSomeWork() from my unit test?

Vue component:

<template>
    <div id="ThisStuff">
        <span>
            Some other stuff is going on here
        </span>
    </div>
</template>

<script>
    import foo from 'bar'

    export default {
        props: {
            ObjectWithStuffInIt: [
                {
                    id: 1
                    bar: false
                },
                {
                    id: 2
                    bar: false
                },
            ]
        },
        data: {
            foo: "foo"
        },
        methods: {
            doSomeWork: function() {
                for (var i = 0; i < ObjectWithStuffInIt.length; i++) { 
                    if (foo === "diddly") {
                        ObjectWithStuffInIt[i].bar = true;
                    }
                }
            }
        }
    }
</script>

My test code:

import {createLocalVue, shallow} from 'vue-test-utils'
import ThisVueFile.test.js from '../../thisPlace/ThatPlace/ThisVueFile.vue'
import Vuex from 'vuex'

const localVue = createLocalVue()
localVue.use(Vuex);

describe('ThisVueFile.test.js', () => {
    let user;
    let store;

    beforeEach(() => {
        let getters = {
            user: () => user
        }

        store = new Vuex.Store({ getters })
    })

    // I need to fill propsData: with some local data here 
    //     because it is server data
    // I need to have access to the method
    // I need to use local data for `foo` in the test. 

    it(' When foo is set to -diddly- then set bar to true ', () => {
        foo = "diddly";
        // run the method in the component here 
        doSomeWork();

        expect(OjbectWithStuffInIt[0].bar.equals(true));
    })
})

Answer 1

Calling component method

The wrapper provides access to the component instance via its vm property, so you could call the method directly with:

wrapper.vm.doSomeWork()

Setting props

• The mounting options (passed to shallowMount() or mount()) include the propsData property that could be used to initialize the component's props before mounting.
• You could also use the wrapper's setProps() after the component has already been mounted.

Example:

it('...', () => {
  const wrapper = shallowMount(MyComponent, {
    propsData: {
      myItems: [
        { id: 200, bar: false },
        { id: 300, bar: false }
      ]
    }
  });

  // OR
  wrapper.setProps({
    myItems: [
      { id: 400: bar: true }
    ]
  })
})

Modifying component data property

• The mounting options includes the data property that could be used to initialize the component's data before mounting.
• You could also use the wrapper's setData() after the component has already mounted.
• You could access the component's data property directly through the wrapper's vm property.

Example:

it('...', () => {
  const wrapper = shallowMount(MyComponent, {
    data() {
      return {
        foo: 1
      }
    }
  });

  // OR
  wrapper.setData({ foo: 2 })

  // OR
  wrapper.vm.foo = 3
})

Full example

Altogether, your test might look similar to this:

import { createLocalVue, shallowMount } from '@vue/test-utils'
import MyComponent from '@/components/MyComponent'

describe('MyComponent', () => {
  it('When foo is set to -something-, set bar to true', () => {
    const myItems = [
      { id: 200, bar: false },
      { id: 300, bar: false }
    ]
    const localVue = createLocalVue()
    const wrapper = shallowMount(MyComponent, {
      localVue,
      propsData: {
        myItems
      }
    })

    wrapper.vm.foo = 'something'
    wrapper.vm.doSomeWork()

    expect(myItems[0].bar).toBe(true)
  })
})

demo