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Testing Vue Components with Emitted Events

Emitted events, like props, are strictly part of the component's API. The end user of your application is not even aware of the concept of emitted events. This means that when you're testing emitted events, the user you should keep in mind while writing the test is the developer who will use your component.

You want to test the API contract of the component -- in Vue, a component's API consists of props, slots, events, and if necessary the surrounding component hierarchy.

Now, when you interact with the component, you should still do so as a user would; however your assertions are focused on the developer's expectations. When you interact with the component, does that component emit the correct events, at the correct time, with the correct arguments?

Testing Emitted Events

In the Stepper component, we bind native DOM click listeners with callbacks to buttons that increment and decrement the Stepper's internal counter.

Because all of the state is managed internally, it is opaque to the developer or parent component consuming the Stepper.

<button aria-label="decrement" @click="counter--">-</button>
{{ counter }}
<button aria-label="increment" @click="counter++">+</button>

This can be fine, but depending on the needs of the developer, it can be difficult for the consumer of the Stepper (e.g. other components) to listen to when change occurs or when the user interacts with the Stepper's various buttons.

One solution is to emit an event called change to the consuming component with the new internal state of the Stepper.

You would use the <Stepper> from a parent component like so:

<div>
  What's your age?
  <Stepper @change="onAgeChange" />
  <!-- onAgeChange is a method the parent component defines -->
</div>

This is what the implementation would look like:

<template>
  <button aria-label="decrement" @click="decrement">-</button>
  {{ counter }}
  <button aria-label="increment" @click="increment">+</button>
</TabItem>

<script setup>
  import { ref } from 'vue'

  const emit = defineEmits(['change'])

  const counter = ref(0)

  const increment = () => {
    counter.value++
    emit('change', counter.value)
  }

  const decrement = () => {
    counter.value--
    emit('change', counter.value)
  }
</script>

As the developer of the Stepper component, you want to make sure that when the end user clicks the increment and decrement buttons, that the change event is emitted to the consuming component.

We suggest using spies to accomplish this. It is still possible to use Vue Test Util's own emitted() API, but the Vue Test Utils developers recommend against doing so and would really rather you use the spy-based API Cypress ships with.

Using Spies​

How do we test that the custom change event is firing the incremented and decremented values for the Stepper? We can use spies when we Arrange, Act, and Assert in our test.

Arrange​

First, we Arrange our test.

  1. Let's setup the spies.
it("clicking increment fires a change event with the incremented value", () => {
  const onChangeSpy = cy.spy().as("changeSpy");
  // ...
});

it("clicking decrement fires a change event with the decremented value", () => {
  const onChangeSpy = cy.spy().as("changeSpy");
  // ...
});

We're aliasing the spy with cy.as('aliasName') so that the Cypress Reporter prints out the name of the spy any time it is invoked. This lets you visually inspect the arguments of the emitted event in your browser.

  1. Let's mount the component with the spies bound.
it("clicking + fires a change event with the incremented value", () => {
  const onChangeSpy = cy.spy().as("changeSpy");
  cy.mount(Stepper, { props: { onChange: onChangeSpy } });
});

You may notice the syntax below relies on binding events to the props key in mount. While this isn't "idiomatic Vue", it's the current signature of Vue Test Utils.

In the future, Cypress may propose an API change to Vue Test Utils so that this syntax feels more natural, because onChange isn't actually a prop -- it's an event.

As such, JSX may feel more idiomatic. At Cypress, we use TSX for all of our Vue tests.

Act​

Next, we Act by firing a click event for the increment button.

const incrementSelector = "[aria-label=increment]";

it("clicking + fires a change event with the incremented value", () => {
  const onChangeSpy = cy.spy().as("changeSpy");
  cy.mount(Stepper, { props: { onChange: onChangeSpy } })
    .get(incrementSelector)
    .click();
});

Assert​

Finally, we Assert that the change event was emitted with the correct value.

const incrementSelector = "[aria-label=increment]";

it("clicking + fires a change event with the incremented value", () => {
  const onChangeSpy = cy.spy().as("changeSpy");
  cy.mount(Stepper, { props: { onChange: onChangeSpy } })
    .get(incrementSelector)
    .click()
    .get("@changeSpy")
    .should("have.been.calledWith", 1);
});

We may decide to combine this test with the previous tests we've written that test multiple things at once in a given scenario.

This is up to the discretion of the developer. Combining tests will result in a faster overall test run, however it may be more difficult to isolate why a test failed in the first place. For end-to-end tests, because setup and visiting pages is expensive, we recommend having longer tests. This is not necessarily a problem for Component tests because they are comparatively quick.

Using Vue Test Utils​

In order to encourage interoperability between your existing component tests and Cypress, we support using Vue Test Utils' API.

cy.mount(Stepper).then((wrapper) => {
  // this is the Vue Test Utils wrapper
});

If you intend to use the wrapper frequently and use Vue Test Util's API, we recommend you write a custom mount command and create a Cypress alias to get back at the wrapper.

import { mount } from "cypress/vue";

Cypress.Commands.add("mount", (...args) => {
  return mount(...args).then((wrapper) => {
    return cy.wrap(wrapper).as("vue");
  });
});

// the "@vue" alias will now work anywhere
// after you've mounted your component
cy.mount(Stepper).doStuff().get("@vue"); // The subject is now the Vue Wrapper

This means that you are able to get to the resulting wrapper returned from the mount command and use wrapper.emitted() in order to gain access to Native DOM events that were fired, as well as Custom Events that were emitted by your component under test.

Because wrapper.emitted() is only data, and NOT spy-based you will have to unpack its results to write assertions.

Your test failure messages will not be as helpful because you're not able to use the Sinon-Chai library that Cypress ships, which comes with methods such as to.have.been.called and to.have.been.calledWith.

Usage of the cy.get('@vue') alias may look something like the below code snippet.

Notice that we're using the 'should' function signature in order to take advantage of Cypress's retryability . If we chained using cy.then instead of cy.should, we may run into the kinds of issues you have in Vue Test Utils tests where you have to use await frequently in order to make sure the DOM has updated or any reactive events have fired.

cy.mount(Stepper, { props: { initial: 100 } })
  .get(incrementSelector)
  .click()
  .get("@vue")
  .should((wrapper) => {
    expect(wrapper.emitted("change")).to.have.length;
    expect(wrapper.emitted("change")[0][0]).to.equal("101");
  });

Regardless of our recommendation to use spies instead of the internal Vue Test Utils API, you may decide to continue using emitted as it automatically records every single event emitted from the component, and so you won't have to create a spy for every event emitted.

This auto-spying behavior could be useful for components that emit many custom events.

What's Next?​

We're going to create a container component and learn how to test slots.