# Mocking global objects

vue-test-utils provides a simple way to mock global objects attached to Vue.prototype, both on test by test basis and to set a default mock for all tests.

The test used in the following example can be found here.

# The mocks mounting option

The mocks mounting option is one way to set the value of any properties attached to Vue.prototype. This commonly includes:

  • $store, for Vuex
  • $router, for Vue Router
  • $t, for vue-i18n

and many others.

# Example with vue-i18n

Use with Vuex and Vue Router are discussed in the respective sections, here and here. Let's see an example with vue-i18n. While it would be possible to use createLocalVue and install vue-i18n for each test, that would quickly get cumbersome and introduce a lot of boilerplate. First, a <Bilingual> component that uses vue-i18n:

<template>
  <div class="hello">
    {{ $t("helloWorld") }}
  </div>
</template>

<script>
  export default {
    name: "Bilingual"
  }
</script>

The way vue-i18n works is you declare your translation in another file, then reference them with $t. For the purpose of this test it doesn't really matter what the translation file looks like, but for this component it could look like this:

export default {
  "en": {
    helloWorld: "Hello world!"
  },
  "ja": {
    helloWorld: "こんにちは、世界!"
  }
}

Based on the locale, the correct translation is rendered. Let's try and render the component in a test, without any mocking.

import { mount } from "@vue/test-utils"
import Bilingual from "@/components/Bilingual.vue"

describe("Bilingual", () => {
  it("renders successfully", () => {
    const wrapper = mount(Bilingual)
  })
})

Running this test with yarn test:unit throws a huge stack trace. If you look through the output carefully, you can see:

[Vue warn]: Error in config.errorHandler: "TypeError: _vm.$t is not a function"

This is because we did not install vue-i18n, so the global $t method does not exist. Let's mock it using the mocks mounting option:

import { mount } from "@vue/test-utils"
import Bilingual from "@/components/Bilingual.vue"

describe("Bilingual", () => {
  it("renders successfully", () => {
    const wrapper = mount(Bilingual, {
      mocks: {
        $t: (msg) => msg
      }
    })
  })
})

Now the test passes! There are lots of uses for the mocks option. Most frequently I find myself mocking the global objects provided by the three packages mentioned above.

# Settings default mocks using config

Sometimes you want to have a default value for the mock, so you don't create it on a test by test basis. You can do this using the config API provided by vue-test-utils. Let's expand the vue-i18n example. You can set default mocks anywhere by doing the following:

import { config } from "@vue/test-utils"

config.mocks["mock"] = "Default Mock Value"

The demo project for this guide is using Jest, so I will declare the default mock in jest.init.js, which is loaded before the tests are run automatically. I will also import the example translations object from earlier, and use it in the mock implementation.

import VueTestUtils from "@vue/test-utils"
import translations from "./src/translations.js"

const locale = "en"

VueTestUtils.config.mocks["$t"] = (msg) => translations[locale][msg]

Now a real translation will be rendered, despite using a mocked $t function. Run the test again, this time using console.log on wrapper.html() and removing the mocks mounting option:

describe("Bilingual", () => {
  it("renders successfully", () => {
    const wrapper = mount(Bilingual)

    console.log(wrapper.html())
  })
})

The test passes, and the following markup is rendered:

<div class="hello">
  Hello world!
</div>

You can read about using mocks to test Vuex here. The technique is the same.

# Conclusion

This guide discussed:

  • using mocks to mock a global object on a test by test basis
  • using config.mocks to set a default mock