Unit Testing Vue-Router
Since a router usually involves multiple components operating together, often routing tests take place further up the testing pyramid, right up at the e2e/integration test level. However, having some unit tests around your routing can be beneficial as well.
The source code for the test described in this article can be found here. There are two ways to test components that interact with a router:
- Using an real router instance
- Mocking the
Since most Vue applications use the official Vue Router, this guide will focus that.
Creating the Components
We will build a simple
<App>, that has a
/nested-child route. Visiting
/nested-child renders a
<NestedRoute> component. Create an
App.vue file, and insert the following minimal component:
<NestedRoute> is equally as minimal:
Nowe we need some routes:
In a real app, you normally would create a
router.js file and import the routes we made, and write something like this:
Since we do not want to polluate the global namespace by calling
Vue.use(...) in our tests, we will create the router on a test by test basis. This will let us have more fine grained control over the state of the application during the unit tests.
Writing the Test
Let’s look at some code, then talk about what’s going on. We are testing
App.vue, so in
App.spec.js add the following:
As usual, we start by importing the various modules for the test. Notably, we are importing the actual routes we will be using for the application. This is ideal in some ways — if the real routing breaks, the unit tests should fail, letting us fix the problem before deploying the application.
We can use the same
localVue for all the
<App> tests, so it is declared outside the first
describe block. However, since we might like to have different tests for different routes, the router is defined inside the
Another notable point that is different from other guides in this book is we are using
mount instead of
shallowMount. If we use
<router-link> will be stubbed out, regardless of the current route, a useless stub component will be rendered.
Workaround for large render trees using
mount is fine in some cases, but sometimes it is not ideal. For example, if you are rendering your entire
<App> component, chances are the render tree is large, containing many components with their own children components and so on. A lot of children components will trigger various lifecycle hooks, making API requests and the such.
If you are using Jest, its powerful mocking system provides an elegent solution to this problem. You can simply mock the child components, in this case
<NestedRoute>. The following mock can be used and the above test will still pass:
Using a Mock Router
Sometimes a real router is not necessary. Let’s update
<NestedRoute> to show a username based on the current path's query string. This time we will use TDD to implement the feature. Here is a basic test that simply renders the component and makes an assertion:
We don’t have a
<div class="username"> yet, so running the test gives us:
Now the test fails with:
This is because
$route does not exist. We could use a real router, but in this case it is easier to just use the
mocks mounting option:
Now the test passes. In this case, we don’t do any navigation or anything that relies on the implementation of the router, so using
mocks is good. We don't really care how
username comes to be in the query string, only that it is present.
Often the server will provide the routing, as opposed to client side routing with Vue Router. In such cases, using
mocks to set the query string in a test is a good alternative to using a real instance of Vue Router.
Stategies for Testing Router Hooks
Vue Router provides several types of router hooks, called “navigation guards”. Two such examples are:
- Global guards (
router.beforeEach). Declared on the router instance.
- In component guards, such as
beforeRouteEnter. Declared in components.
Making sure these behavae correctly is usually a job for an integration test, since you need to have a user navigate from one route to another. However, you can also use unit tests to see if the functions called in the navigation guards are working correctly and get faster feedback about potential bugs. Here are some strategies on decouple logic from nagivation guards, and writing unit tests around them.
Let’s say you have a
bustCache function that should be called on every route that contains the
shouldBustCache meta field. You routes might look like this:
shouldBustCache meta field, you want to invalidate the current cache to ensure the user does not get stale data. An implementation might look like this:
In your unit test, you could import the router instance, and attempt to call
beforeEach by typing
router.beforeHooks(). This will throw an error about
next - since you didn't pass the correct arguments. Instead of this, one strategy is to decouple and independently export the
beforeEach navigation hook, before coupling it to the router. How about:
Now writing a test is easy, albeit a little long:
The main point of interest is we mock the entire module using
jest.mock, and reset the mock using the
afterEach hook. By exporting the
To ensure the hook is actually calling
bustCache and showing the most recent data, a e2e testing tool like Cypress.io, which comes with applications scaffolded using vue-cli, can be used.
beforeRouteLeave hook to
We can test this in exactly the same way as the global guard:
While this style of unit test can be useful for immediate feedback during development, since routers and navigation hooks often interact with several components to achieve some effect, you should also have integration tests to ensure everything is working as expected.
This guide covered:
- testing components conditionally rendered by Vue Router
- mocking Vue components using
- decoupling global navigation guards from the router and testing the independently
jest.mockto mock a module
Originally published in the Vue Testing Handbook.