Making HTTP requests

Modern test runners already provide lots of great features when it comes to test HTTP requests. Thus, Vue Test Utils doesn't feature any unique tool to do so.

However, it is an important feature to test, and there are a few gotchas we want to highlight.

In this section, we explore some patterns to perform, mock, and assert HTTP requests.

A list of blog posts

Let's start with a basic use case. The following PostList component renders a list of blog posts fetched from an external API. To get these posts, the component features a button element that triggers the request:

<template>
  <button @click="getPosts">Get posts</button>
  <ul>
    <li v-for="post in posts" :key="post.id" data-test="post">
      {{ post.title }}
    </li>
  </ul>
</template>

<script>
import axios from 'axios'

export default {
  data() {
    return {
      posts: null
    }
  },
  methods: {
    async getPosts() {
      this.posts = await axios.get('/api/posts')
    }
  }
}
</script>

There are several things we need to do to test this component properly.

Our first goal is to test this component without actually reaching the API. This would create a fragile and potentially slow test.

Secondly, we need to assert that the component made the right call with the appropriate parameters. We won't be getting results from that API, but we still need to ensure we requested the right resources.

Also, we need to make sure that the DOM has updated accordingly and displays the data. We do so by using the flushPromises() function from @vue/test-utils.

import { mount, flushPromises } from '@vue/test-utils'
import axios from 'axios'
import PostList from './PostList.vue'

const mockPostList = [
  { id: 1, title: 'title1' },
  { id: 2, title: 'title2' }
]

// Following lines tell Jest to mock any call to `axios.get`
// and to return `mockPostList` instead
jest.spyOn(axios, 'get').mockResolvedValue(mockPostList)

test('loads posts on button click', async () => {
  const wrapper = mount(PostList)

  await wrapper.get('button').trigger('click')

  // Let's assert that we've called axios.get the right amount of times and
  // with the right parameters.
  expect(axios.get).toHaveBeenCalledTimes(1)
  expect(axios.get).toHaveBeenCalledWith('/api/posts')

  // Wait until the DOM updates.
  await flushPromises()

  // Finally, we make sure we've rendered the content from the API.
  const posts = wrapper.findAll('[data-test="post"]')

  expect(posts).toHaveLength(2)
  expect(posts[0].text()).toContain('title1')
  expect(posts[1].text()).toContain('title2')
})

Pay attention that we added prefix mock to the variable mockPostList. If not, we will get the error: "The module factory of jest.mock() is not allowed to reference any out-of-scope variables.". This is jest-specific, and you can read more about this behavior in their docs.

Also notice how we awaited flushPromises and then interacted with the Component. We do so to ensure that the DOM has been updated before the assertions run.

Alternatives to jest.mock()

There are several ways of setting mocks in Jest. The one used in the example above is the simplest. For more powerful alternatives, you might want to check out axios-mock-adapter or msw, among others.

Asserting loading state

Now, this PostList component is pretty useful, but it lacks some other awesome features. Let's expand it to make it display a fancy message while loading our posts!

Also, let's disable the <button> element while loading, too. We don't want users to keep sending requests while fetching!

<template>
  <button :disabled="loading" @click="getPosts">Get posts</button>

  <p v-if="loading" role="alert">Loading your posts…</p>
  <ul v-else>
    <li v-for="post in posts" :key="post.id" data-test="post">
      {{ post.title }}
    </li>
  </ul>
</template>

<script>
import axios from 'axios'

export default {
  data() {
    return {
      posts: null,
      loading: null
    }
  },
  methods: {
    async getPosts() {
      this.loading = true

      this.posts = await axios.get('/api/posts')

      this.loading = null
    }
  }
}
</script>

Let's write a test to assert that all the loading-related elements are rendered on time.

test('displays loading state on button click', async () => {
  const wrapper = mount(PostList)

  // Notice that we run the following assertions before clicking on the button
  // Here, the component should be in a "not loading" state.
  expect(wrapper.find('[role="alert"]').exists()).toBe(false)
  expect(wrapper.get('button').attributes()).not.toHaveProperty('disabled')

  // Now let's trigger it as usual.
  await wrapper.get('button').trigger('click')

  // We assert for "Loading state" before flushing all promises.
  expect(wrapper.find('[role="alert"]').exists()).toBe(true)
  expect(wrapper.get('button').attributes()).toHaveProperty('disabled')

  // As we did before, wait until the DOM updates.
  await flushPromises()

  // After that, we're back at a "not loading" state.
  expect(wrapper.find('[role="alert"]').exists()).toBe(false)
  expect(wrapper.get('button').attributes()).not.toHaveProperty('disabled')
})

HTTP requests from Vuex

A typical scenario for more complex applications is to trigger a Vuex action that performs the HTTP request.

This is no different from the example outlined above. We might want to load the store as is and mock services such as axios. This way, we're mocking our system's boundaries, thus achieving a higher degree of confidence in our tests.

You can check out the Testing Vuex docs for more information on testing Vuex with Vue Test Utils.

Conclusion

• Vue Test Utils does not require special tools to test HTTP requests. The only thing to take into account is that we're testing asynchronous behavior.
• Tests must not depend on external services. Use mocking tools such as jest.mock to avoid it.
• flushPromises() is a useful tool to make sure the DOM updates after an async operation.
• Directly triggering HTTP requests by interacting with the component makes your test more resilient.