FAQ

See also the main FAQ for questions not specific to React testing

How do I test input onChange handlers?

TL;DR: Go to the on-change.js example

In summary:

import React from 'react'
import {render, fireEvent} from '@testing-library/react'
test('change values via the fireEvent.change method', () => {
const handleChange = jest.fn()
const {container} = render(<input type="text" onChange={handleChange} />)
const input = container.firstChild
fireEvent.change(input, {target: {value: 'a'}})
expect(handleChange).toHaveBeenCalledTimes(1)
expect(input.value).toBe('a')
})
test('select drop-downs must use the fireEvent.change', () => {
const handleChange = jest.fn()
const {container} = render(
<select onChange={handleChange}>
<option value="1">1</option>
<option value="2">2</option>
</select>,
)
const select = container.firstChild
const option1 = container.getElementsByTagName('option').item(0)
const option2 = container.getElementsByTagName('option').item(1)
fireEvent.change(select, {target: {value: '2'}})
expect(handleChange).toHaveBeenCalledTimes(1)
expect(option1.selected).toBe(false)
expect(option2.selected).toBe(true)
})
test('checkboxes (and radios) must use fireEvent.click', () => {
const handleChange = jest.fn()
const {container} = render(<input type="checkbox" onChange={handleChange} />)
const checkbox = container.firstChild
fireEvent.click(checkbox)
expect(handleChange).toHaveBeenCalledTimes(1)
expect(checkbox.checked).toBe(true)
})

If you've used enzyme or React's TestUtils, you may be accustomed to changing inputs like so:

input.value = 'a'
Simulate.change(input)

We can't do this with React Testing Library because React actually keeps track of any time you assign the value property on an input and so when you fire the change event, React thinks that the value hasn't actually been changed.

This works for Simulate because they use internal APIs to fire special simulated events. With React Testing Library, we try to avoid implementation details to make your tests more resilient.

So we have it worked out for the change event handler to set the property for you in a way that's not trackable by React. This is why you must pass the value as part of the change method call.

Can I write unit tests with this library?

Definitely yes! You can write unit and integration tests with this library. See below for more on how to mock dependencies (because this library intentionally does NOT support shallow rendering) if you want to unit test a high level component. The tests in this project show several examples of unit testing with this library.

As you write your tests, keep in mind:

The more your tests resemble the way your software is used, the more confidence they can give you. - 17 Feb 2018

If I can't use shallow rendering, how do I mock out components in tests?

In general, you should avoid mocking out components (see the Guiding Principles section). However if you need to, then it's pretty trivial using Jest's mocking feature. One case that I've found mocking to be especially useful is for animation libraries. I don't want my tests to wait for animations to end.

jest.mock('react-transition-group', () => {
const FakeTransition = jest.fn(({children}) => children)
const FakeCSSTransition = jest.fn(props =>
props.in ? <FakeTransition>{props.children}</FakeTransition> : null,
)
return {CSSTransition: FakeCSSTransition, Transition: FakeTransition}
})
test('you can mock things with jest.mock', () => {
const {getByTestId, queryByTestId} = render(
<HiddenMessage initialShow={true} />,
)
expect(queryByTestId('hidden-message')).toBeTruthy() // we just care it exists
// hide the message
fireEvent.click(getByTestId('toggle-message'))
// in the real world, the CSSTransition component would take some time
// before finishing the animation which would actually hide the message.
// So we've mocked it out for our tests to make it happen instantly
expect(queryByTestId('hidden-message')).toBeNull() // we just care it doesn't exist
})

Note that because they're Jest mock functions (jest.fn()), you could also make assertions on those as well if you wanted.

Open full test for the full example.

This looks like more work than shallow rendering (and it is), but it gives you more confidence so long as your mock resembles the thing you're mocking closely enough.

If you want to make things more like shallow rendering, then you could do something more like this.

Learn more about how Jest mocks work from my blog post: "But really, what is a JavaScript mock?"

What about enzyme is "bloated with complexity and features" and "encourage poor testing practices"?

Most of the damaging features have to do with encouraging testing implementation details. Primarily, these are shallow rendering, APIs which allow selecting rendered elements by component constructors, and APIs which allow you to get and interact with component instances (and their state/properties) (most of enzyme's wrapper APIs allow this).

The guiding principle for this library is:

The more your tests resemble the way your software is used, the more confidence they can give you. - 17 Feb 2018

Because users can't directly interact with your app's component instances, assert on their internal state or what components they render, or call their internal methods, doing those things in your tests reduce the confidence they're able to give you.

That's not to say that there's never a use case for doing those things, so they should be possible to accomplish, just not the default and natural way to test react components.

Why isn't snapshot diffing working?

If you use the snapshot-diff library to save snapshot diffs, it won't work out of the box because this library uses the DOM which is mutable. Changes don't return new objects so snapshot-diff will think it's the same object and avoid diffing it.

Luckily there's an easy way to make it work: clone the DOM when passing it into snapshot-diff. It looks like this:

const firstVersion = container.cloneNode(true)
// Do some changes
snapshotDiff(firstVersion, container.cloneNode(true))
How do I fix "an update was not wrapped in act(...)" warnings?

This warning is usually caused by an async operation causing an update after the test has already finished. There are 2 approaches to resolve it:

  1. Wait for the result of the operation in your test by using one of the async utilities like waitFor or a find* query. For example: const userAddress = await findByLabel(/address/i).
  2. Mocking out the asynchronous operation so that it doesn't trigger state updates.

Generally speaking, approach 1 is preferred since it better matches the expectations of a user interacting with your app.

In addition, you may find this blog post helpful as you consider how best to write tests that give you confidence and avoid these warnings.

What level of a component tree should I test? Children, parents, or both?

Following the guiding principle of this library, it is useful to break down how tests are organized around how the user experiences and interacts with application functionality rather than around specific components themselves. In some cases, for example for reusable component libraries, it might be useful to include developers in the list of users to test for and test each of the reusable components individually. Other times, the specific break down of a component tree is just an implementation detail and testing every component within that tree individually can cause issues (see https://kentcdodds.com/blog/avoid-the-test-user).

In practice this means that it is often preferable to test high enough up the component tree to simulate realistic user interactions. The question of whether it is worth additionally testing at a higher or lower level on top of this comes down to a question of tradeoffs and what will provide enough value for the cost (see https://kentcdodds.com/blog/unit-vs-integration-vs-e2e-tests on more info on different levels of testing).

For a more in-depth discussion of this topic see this video.

Last updated on by Nick McCurdy