Testing Dom events in Vue.js using Jest and vue-test-utils

In this tutorial, we are going to learn about how to test native dom events in vue using jest and vue-test-utils library.

Native dom events are happening in the browser when we click on button or hovering a mouse on the element, In Vue.js we can respond to the dom events by adding event handler methods to the element by using v-on directive or @ syntax.

The example component we are testing.

<template>
  <div>
    <h1>{{count}}</h1>
    <button @click="increment">Increment</button>  </div>
</template>

<script>
export default {
  data: function() {
    return {
      count:0
    };
  },
  methods: {
    increment() {
      this.count++;
    }
  }
};
</script>

In the above component, we created a counter which increments it’s count value by clicking a button.

Writing tests for dom events

In vue-test-utils, every wrapper object contains a trigger method by using that we can trigger dom events.

Let’s write tests for our Counter component.

import { shallowMount } from '@vue/test-utils';
import Post from '../src/components/Counter.vue'
describe('Testing native dom events', () => {
    const wrapper = shallowMount(Post);

    it('calls increment method when button is clicked', () => {
        const increment = jest.fn(); // mock function
        // updating method with mock function
        wrapper.setMethods({ increment });
        //find the button and trigger click event
        wrapper.find('button').trigger('click');        expect(increment).toBeCalled();
    })

})

In the above code, we first created a mock function called increment and updated the component method with the mock function by using wrapper.setMethods().

At final we trigger the click event on the button element and asserting that increment mock function was called.