should be present
+- if `error` is `false`, `
should not be present
+
+And our first attempt at at test:
+
+```js
+import { shallow } from 'vue-test-utils'
+
+describe('Foo', () => {
+ it('renders a message and responds correctly to user input', () => {
+ const wrapper = shallow(Foo, {
+ data: {
+ message: 'Hello World',
+ username: ''
+ }
+ })
+
+ // see if the message renders
+ expect(wrapper.find('.message').text()).toEqual('Hello World')
+
+ // assert the error is rendered
+ expect(wrapper.find('.error').exists()).toBeTruthy()
+
+ // update the username and assert error is longer rendered
+ wrapper.setData({ username: 'Lachlan' })
+ expect(wrapper.find('.error').exists()).toBeFalsy()
+ })
+})
+```
+
+There are some problems with the above:
+- a single test is making assertions about different things
+- difficult to tell the different states the component can be in, and what should be rendered
+
+The below example improves the test by:
+- only making one assertion per `it` block
+- having short, clear test descriptions
+- providing only the minimum data requires for the test
+- refactoring duplucated logic (creating the `wrapper` and setting the `username` variable) into a factory function
+
+*Updated test*:
+```js
+import { shallow } from 'vue-test-utils'
+import Foo from './Foo'
+
+const factory = (values = {}) => {
+ return shallow(Foo, {
+ data: { ...values }
+ })
+}
+
+describe('Foo', () => {
+ it('renders a welcome message', () => {
+ const wrapper = factory()
+
+ expect(wrapper.find('.message').text()).toEqual("Welcome to the Vue.js cookbook")
+ })
+
+ it('renders an error when username is less than 7 characters', () => {
+ const wrapper = factory({ username: '' })
+
+ expect(wrapper.find('.error').exists()).toBeTruthy()
+ })
+
+ it('renders an error when username is whitespace', () => {
+ const wrapper = factory({ username: ' '.repeat(7) })
+
+ expect(wrapper.find('.error').exists()).toBeTruthy()
+ })
+
+ it('does not render an error when username is 7 characters or more', () => {
+ const wrapper = factory({ username: 'Lachlan' })
+
+ expect(wrapper.find('.error').exists()).toBeFalsy()
+ })
+})
+```
+
+Points to note:
+
+At the top, we declare the factory function which merges the `values` object into `data` and returns a new `wrapper` instance. This way, we don't need to duplicate `const wrapper = shallow(Foo)` in every test. Another great benefit to this is when more complex components with a method or computed property you might want to mock or stub in every test, you only need to declare it once.
+
+## Additional Context
+
+Thee above test is fairly simple, but in practise Vue components often have other behaviors you want to test, such as:
+
+- making API calls
+- committing or dispatching mutations or actions with a `Vuex` store
+- testing interaction
+
+There are more complete examples showing such tests in the `vue-test-utils` [guides](https://vue-test-utils.vuejs.org/en/guides/).
+
+`vue-test-utils` and the enormous JavaScript ecosystem provides plenty of tooling to facilitate almost 100% test coverage. Unit tests are only one part of the testing pyramid, though. Some other types of tests include e2e (end to end) tests, and snapshot tests. Unit tests are the smallest and most simple of tests - they make assertions on the smallest units of work, isolating each part of a single component.
+
+Snapshot tests save the markup of your Vue component, and compare to the new one generated each time the test runs. If something changes, the developer is notified, and can decide if the change was intentional (the component was updated) or accidentally (the component is behaving incorrectly).
+
+End to end tests involve ensure a number of components interact together well. They are more high level. Some examples might be testing if a user can sign up, log in, and update their username. These are slowly to run than unit tests or snapshot tests.
+
+Unit tests are most useful during development, either to help a developer think about how to design a component, or refactor an existing component, and are often run every time code is changed.
+
+Higher level tests, such as end to end tests, run much slower. These usually run pre-deploy, to ensure each part of the system is working together correctly.
+
+More information about testing Vue components can be found in [Testing Vue.js Applications](https://www.manning.com/books/testing-vuejs-applications) by core team member [Edd Yerburgh](https://eddyerburgh.me/).
+
+## When To Avoid This Pattern
+
+Unit testing is an important part of any serious application. At first, when the vision of an application is not clear, unit testing might slow down development, but once a vision is established and real users will be interacting with the application, unit tests (and other types of automated tests) are absolutely essential to ensure the codebase is maintainable and scalable.