How can I test a custom input Vue component - vue.js

Question

In the Vue.js documentation, there is an example of a custom input component. I'm trying to figure out how I can write a unit test for a component like that. Usage of the component would look like this
<currency-input v-model="price"></currency-input>
The full implementation can be found at https://v2.vuejs.org/v2/guide/components.html#Form-Input-Components-using-Custom-Events
The documentation says
So for a component to work with v-model, it should (these can be configured in 2.2.0+):
accept a value prop
emit an input event with the new value
How do I write a unit test that ensures that I've written this component such that it will work with v-model? Ideally, I don't want to specifically test for those two conditions, I want to test the behavior that when the value changes within the component, it also changes in the model.

Answer 1

You can do it:
Using Vue Test Utils, and
Mounting a parent element that uses <currency-input>
Fake an input event to the inner text field of <currency-input> with a value that it transforms (13.467 is transformed by <currency-input> to 13.46)
Verify if, in the parent, the price property (bound to v-model) has changed.
Example code (using Mocha):
import { mount } from '#vue/test-utils'
import CurrencyInput from '#/components/CurrencyInput.vue'
describe('CurrencyInput.vue', () => {
it("changing the element's value, updates the v-model", () => {
var parent = mount({
data: { price: null },
template: '<div> <currency-input v-model="price"></currency-input> </div>',
components: { 'currency-input': CurrencyInput }
})
var currencyInputInnerTextField = parent.find('input');
currencyInputInnerTextField.element.value = 13.467;
currencyInputInnerTextField.trigger('input');
expect(parent.vm.price).toBe(13.46);
});
});
In-browser runnable demo using Jasmine:
var CurrencyInput = Vue.component('currency-input', {
template: '\
<span>\
$\
<input\
ref="input"\
v-bind:value="value"\
v-on:input="updateValue($event.target.value)">\
</span>\
',
props: ['value'],
methods: {
// Instead of updating the value directly, this
// method is used to format and place constraints
// on the input's value
updateValue: function(value) {
var formattedValue = value
// Remove whitespace on either side
.trim()
// Shorten to 2 decimal places
.slice(0, value.indexOf('.') === -1 ? value.length : value.indexOf('.') + 3)
// If the value was not already normalized,
// manually override it to conform
if (formattedValue !== value) {
this.$refs.input.value = formattedValue
}
// Emit the number value through the input event
this.$emit('input', Number(formattedValue))
}
}
});
// specs code ///////////////////////////////////////////////////////////
var mount = vueTestUtils.mount;
describe('CurrencyInput', () => {
it("changing the element's value, updates the v-model", () => {
var parent = mount({
data() { return { price: null } },
template: '<div> <currency-input v-model="price"></currency-input> </div>',
components: { 'currency-input': CurrencyInput }
});
var currencyInputInnerTextField = parent.find('input');
currencyInputInnerTextField.element.value = 13.467;
currencyInputInnerTextField.trigger('input');
expect(parent.vm.price).toBe(13.46);
});
});
// load jasmine htmlReporter
(function() {
var env = jasmine.getEnv()
env.addReporter(new jasmine.HtmlReporter())
env.execute()
}())
<link rel="stylesheet" type="text/css" href="https://cdn.jsdelivr.net/jasmine/1.3.1/jasmine.css">
<script src="https://cdn.jsdelivr.net/jasmine/1.3.1/jasmine.js"></script>
<script src="https://cdn.jsdelivr.net/jasmine/1.3.1/jasmine-html.js"></script>
<script src="https://npmcdn.com/vue#2.5.15/dist/vue.js"></script>
<script src="https://cdn.jsdelivr.net/npm/vue-template-compiler#2.5.15/browser.js"></script>
<script src="https://rawgit.com/vuejs/vue-test-utils/2b078c68293a41d68a0a98393f497d0b0031f41a/dist/vue-test-utils.iife.js"></script>
Note: The code above works fine (as you can see), but there can be improvements to tests involving v-model soon. Follow this issue for up-to-date info.

Answer 2

I would also mount a parent element that uses the component. Below a newer example with Jest and Vue Test Utils. Check the Vue documentation for more information.
import { mount } from "#vue/test-utils";
import Input from "Input.vue";
describe('Input.vue', () => {
test('changing the input element value updates the v-model', async () => {
const wrapper = mount({
data() {
return { name: '' };
},
template: '<Input v-model="name" />',
components: { Input },
});
const name = 'Brendan Eich';
await wrapper.find('input').setValue(name);
expect(wrapper.vm.$data.name).toBe(name);
});
test('changing the v-model updates the input element value', async () => {
const wrapper = mount({
data() {
return { name: '' };
},
template: '<Input v-model="name" />',
components: { Input },
});
const name = 'Bjarne Stroustrup';
await wrapper.setData({ name });
const inputElement = wrapper.find('input').element;
expect(inputElement.value).toBe(name);
});
});
Input.vue component:
<template>
<input :value="$attrs.value" #input="$emit('input', $event.target.value)" />
</template>