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Introducing the react-testing-library

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Introducing the react-testing-library

In this post, a developer introduces an open source library he has developed, and explains how to use it, and the benfits it brings to React developers.

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Two weeks ago, I wrote a new library! I've been thinking about it for a while. But two weeks ago I started getting pretty serious about it.

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Read on to get an idea of what I mean by "damaging practices."

Simple and complete React DOM testing utilities that encourage good testing practices.

The Problem

You want to write maintainable tests for your React components. As a part of this goal, you want your tests to avoid including implementation details of your components and rather focus on making your tests give you the confidence for which they are intended. As part of this, you want your test base to be maintainable in the long run so refactors of your components (changes to implementation but not functionality) don't break your tests and slow you and your team down.

This Solution

The react-testing-library is a very light-weight solution for testing React components. It provides light utility functions on top of react-dom and react-dom/test-utils, in a way that encourages better testing practices. It's primary guiding principle is:

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So rather than dealing with instances of rendered React components, your tests will work with actual DOM nodes. The utilities this library provides facilitate querying the DOM in the same way the user would. Finding for elements by their label text (just like a user would), finding links and buttons from their text (like a user would). It also exposes a recommended way to find elements by a data-testid as an "escape hatch" for elements where the text content and label do not make sense or is not practical.

This library encourages your applications to be more accessible and allows you to get your tests closer to using your components the way a user will, which allows your tests to give you more confidence that your application will work when a real user uses it.

This library is a replacement for enzyme. While you can follow these guidelines using enzyme itself, enforcing this is harder because of all the extra utilities that enzyme provides (utilities which facilitate testing implementation details). Read more about this in the FAQ.

Also, while the react-testing-library is intended for react-dom, it can support React Native with this short setup file.

What this library is not:

  1. A test runner or framework.
  2. Specific to a testing framework (though we recommend Jest as our preference, the library works with any framework, and even in CodeSandbox!)

Practical Example

import React from 'react'
import {render, Simulate, wait} from 'react-testing-library'
// this adds custom expect matchers
import 'react-testing-library/extend-expect'
// the mock lives in a __mocks__ directory
import axiosMock from 'axios'
import GreetingFetcher from '../greeting-fetcher'
test('displays greeting when clicking Load Greeting', async () => {
  // Arrange
  axiosMock.get.mockImplementationOnce(({name}) =>
      data: {greeting: `Hello ${name}`}
  const {
  } = render(<GreetingFetcher />)
  // Act
  getByLabelText('name').value = 'Mary'
  Simulate.click(getByText('Load Greeting'))
  // let's wait for our mocked `get` request promise to resolve
  // wait will wait until the callback doesn't throw an error
  await wait(() => getByTestId('greeting-text'))
  // Assert
  // here's a custom matcher!
    'Hello Mary'
  // snapshots work great with regular DOM nodes!

The most important takeaway from this example is:

The test is written in such a way that it resembles how the user is using your application.

Let's explore this further...

Our GreetingFetcher might render some HTML like this:

  <label for="name-input">Name</label>
  <input id="name-input" />
  <button>Load Greeting</button>  
  <div data-testid="greeting-text"></div>

So the functionality is: Set the name, click the "Load Greeting" button, and a server request is made to load greeting text with that name.

In your test, you'll need to find the <input /> so you can set its value to something. Conventional wisdom suggests you could use the id property in a CSS selector: #name-input. But is that what the user does to find that input? Definitely not! They look at the screen and find the input with the label "Name" and fill that in. So that's what our test is doing with getByLabelText. It gets the form control based on its label.

Often in tests using enzyme, to find the "Load Greeting" button you might use a CSS selector or even find by the component displayName or the component constructor. But when the user wants to load the greeting, they don't care about those implementation details, instead, they're going to find and click the button that says, "Load Greeting." And that's exactly what our test is doing with the getByText helper!

In addition, the wait resembles exactly what the user does. They wait for the greeting text to appear, however long that takes. In our tests, we're mocking that out so it happens basically instantly, but our test doesn't actually care how long it takes. We don't have to use a setTimeout in our test or anything. We simply say: "Hey, wait until the greeting-text node appears." (Note, in this case, it's using a data-testid attribute which is an escape hatch for situations where it doesn't make sense to find an element by any other mechanism. A data-testidis definitely better than the alternatives.

High-Level Overview API

Originally, the library only provided queryByTestId as a utility as suggested in my blog post "Making your UI tests resilient to change." But thanks to feedback on that blog post from Bergé Greg as well as inspiration from a fantastic (and short!) talk by Jamie White, I added several more and now I'm even happier with this solution.

You can read more about the library and its APIs in the official docs. Here's a high-level overview of what this library gives you:

Simulate: a re-export from the Simulate utility from the react-dom/test-utilsSimulate object.

  • wait: allows you to wait for a non-deterministic period of time in your tests. Normally you should mock out API requests or animations, but even if you're dealing with immediately resolved promises, you'll need your tests to wait for the next tick of the event loop and wait is really good for that. (Big shout out to Ɓukasz Gozda Gandecki who introduced this as a replacement for the (now deprecated)flushPromises API).
  • render: This is the meat of the library. It's fairly simple. It creates a divwith document.createElement, then uses ReactDOM.render to render to that div.

The render function returns the following objects and utilities:

  • container: The div your component was rendered to
  • unmount: A simple wrapper over ReactDOM.unmountComponentAtNodeto unmount your component (to facilitate easier testing of componentWillUnmount for example).
  • getByLabelText: Get a form control associated to a label
  • getByPlaceholderText: Placeholders aren't proper alternatives to labels, but if this makes more sense for your use case it's available.
  • getByText: Get any element by its text content.
  • getByAltText: Get an element (like an <img) by it's alt attribute value.
  • getByTestId: Get an element by its data-testid attribute.

Each of those get* utilities will throw a useful error message if no element can be found. There's also an associated query* API for each which will return nullinstead of throwing an error which can be useful for asserting that an element is not in the DOM.

Also, for these get* utilities, to find a matching element, you can pass:

  • a case-insensitive substring: lo world matches Hello World
  • a regex: /^Hello World$/ matches Hello World
  • a function that accepts the text and the element: (text, el) => el.tagName === 'SPAN' && text.startsWith('Hello') would match a span that has content that starts with Hello

Custom Jest Matchers

Thanks to Anto Aravinth Belgin Rayen, we have some handy custom Jest matchers as well:

  • toBeInTheDOM: Assert whether an element present in the DOM or not.
  • toHaveTextContent: Check whether the given element has a text content or not.
  • Conclusion

    A big feature of this library is that it doesn't have utilities that enable testing implementation details. It focuses on providing utilities that encourage good testing and software practices. I hope that by using the react-testing-libraryyour React test bases are easier to understand and maintain.

    • Oh, by the way, I was originally going to call this library: react-dom-testing-assistant or rdta, but after I tweeted about it, Ryan suggestedreact-testing-library and I couldn't pass it up.
    open source ,open source library ,react.js ,javascript library

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