Write the first Vue.js component unit test in Jest

By: Alex Jover Morales

vue-test-utils, the official VueJS testing library and based on avoriaz, is just around the corner. @EddYerburgh is indeed doing a very good job creating it. It provides all necessary tooling for making easy to write unit test in a VueJS application.

Jest, on the other side, is the testing framework developed at Facebook, which makes testing a breeze, with awesome features such as:

  • Almost no config by default
  • Very cool interactive mode
  • Run tests in parallel
  • Spies, stubs and mocks out of the box
  • Built in code coverage
  • Snapshot testing
  • Module mocking utilities

Probably you've already written tests without this tool, and just by using karma + mocha + chai + sinon + ..., but you'll see how much easier it can be 😉.

Set up a vue-test sample project

Let's start by creating a new project using vue-cli answering NO to all yes/no questions:

npm install -g vue-cli
vue init webpack vue-test
cd vue-test

Then we'll need to install some dependencies:

# Install dependencies
npm i -D jest vue-jest babel-jest

jest-vue-preprocessor is needed for making jest understand .vue files, and babel-jest for the integration with Babel.

vue-test-utils can be installed from npm.

npm i -D vue-test-utils

Let's add the following Jest configuration in the package.json:

...
"jest": {
  "moduleNameMapper": {
    "^vue$": "vue/dist/vue.common.js"
  },
  "moduleFileExtensions": [
    "js",
    "vue"
  ],
  "transform": {
    "^.+\\.js$": "<rootDir>/node_modules/babel-jest",
    ".*\\.(vue)$": "<rootDir>/node_modules/vue-jest"
  }
}
...

moduleFileExtensions will tell Jest which extensions to look for, and transform which preprocessor to use for a file extension.

At last, add a test script to the package.json:

{
  "scripts": {
    "test": "jest",
    ...
  },
  ...
}

Testing a component

I'll be using Single File Components here, and I haven't checked if it works by splitting them in their own html, css or js files, so let's assume you're doing that as well.

First create a MessageList.vue component under src/components:

<template>
    <ul>
        <li v-for="message in messages">
            {{ message }}
        </li>
    </ul>
</template>

<script>
export default {
  name: 'list',
  props: ['messages']
}
</script>

Any update App.vue to use it, as follows:

<template>
  <div id="app">
    <MessageList :messages="messages"/>
  </div>
</template>

<script>
import MessageList from './components/MessageList'

export default {
  name: 'app',
  data: () => ({ messages: ['Hey John', 'Howdy Paco'] }),
  components: {
    MessageList
  }
}
</script>

We have already a couple of components that we can test. Let's create a test folder under the project root, and a App.test.js:

import Vue from "vue";
import App from "../src/App";

describe("App.test.js", () => {
  let cmp, vm;

  beforeEach(() => {
    cmp = Vue.extend(App); // Create a copy of the original component
    vm = new cmp({
      data: {
        // Replace data value with this fake data
        messages: ["Cat"]
      }
    }).$mount(); // Instances and mounts the component
  });

  it('equals messages to ["Cat"]', () => {
    expect(vm.messages).toEqual(["Cat"]);
  });
});

Right now, if we run npm test (or npm t as a shorthand version), the test should run and pass. Since we're modifying the tests, let's better run it in watch mode:

npm t -- --watch

The problem with nested components

This test is too simple. Let's check that the output is the expected as well. For that we can use the amazing Snapshots feature of Jest, that will generate a snapshot of the output and check it against in the upcoming runs. Add after the previous it in App.test.js:

it("has the expected html structure", () => {
  expect(vm.$el).toMatchSnapshot();
});

That will create a test/__snapshots__/App.test.js.snap file. Let's open it and inspect it:

// Jest Snapshot v1, https://goo.gl/fbAQLP

exports[`App.test.js has the expected html structure 1`] = `
<div
  id="app"
>
  <ul>
    <li>
      Cat
    </li>
  </ul>
</div>
`;

In case you haven't noticed, there is a big problem here: the MessageList component has been rendered as well. Unit tests must be tested as an independent unit, meaning that in App.test.js we wanna test App component and don't care at all about anything else.

This can be the reason of several problems. Imagine for example, that the children components (MessageList in this case) perform side effect operations on the created hook, such as calling fetch, a Vuex action or state changes? That's something we definitely don't want.

Luckily, Shallow Rendering solves this nicely.

What is shallowing rendering?

Shallow Rendering is a technique that assures your component is rendering without children. This is useful for:

  • Testing only the component you want to test (that's what Unit Test stands for)
  • Avoid side effects that children components can have, such as making HTTP calls, calling store actions...

Testing a component with vue-test-utils

vue-test-utils provide us with Shallow Rendering among other features. We could rewrite the previous test as follows:

import { shallowMount } from "@vue/test-utils";
import App from "../src/App";

describe("App.test.js", () => {
  let cmp;

  beforeEach(() => {
    cmp = shallowMount(App, {
      // Create a shallow instance of the component
      data: {
        messages: ["Cat"]
      }
    });
  });

  it('equals messages to ["Cat"]', () => {
    // Within cmp.vm, we can access all Vue instance methods
    expect(cmp.vm.messages).toEqual(["Cat"]);
  });

  it("has the expected html structure", () => {
    expect(cmp.element).toMatchSnapshot();
  });
});

And now, if you're still running Jest in watching mode, you'll see the test still pass, but the Snapshot doesn't match. Press u to regenerate it. Open and inspect it again:

// Jest Snapshot v1, https://goo.gl/fbAQLP

exports[`App.test.js has the expected html structure 1`] = `
<div
  id="app"
>
  <!--  -->
</div>
`;

You see? Now no children have been rendered and we tested the App component fully isolated from the component tree. Also, if you have any created or whatever hooks in the children components, they haven't been called either 😉.

If you're curious about how shallow render is implemented, check out the source code and you'll see that basically is stubbing the components key, the render method and the lifecycle hooks.

In the same vein, you can implement the MessageList.test.js test as follows:

import { mount } from "@vue/test-utils";
import MessageList from "../src/components/MessageList";

describe("MessageList.test.js", () => {
  let cmp;

  beforeEach(() => {
    cmp = mount(MessageList, {
      // Be aware that props is overridden using `propsData`
      propsData: {
        messages: ["Cat"]
      }
    });
  });

  it('has received ["Cat"] as the message property', () => {
    expect(cmp.vm.messages).toEqual(["Cat"]);
  });

  it("has the expected html structure", () => {
    expect(cmp.element).toMatchSnapshot();
  });
});

Find the full example on Github.

If you'd like to take a deeper dive into testing Vue components with Jest, then check out Alex's course, "Testing Vue components with Jest". You learn the basics of unit tests in Jest and then build up to more intricate tests of different Vue components.

By: Alex Jover Morales
LinkedIn: https://www.linkedin.com/in/alex-jover-morales-994752102/

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