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In order to use React Testing Library, we will need to include the @testing-library/react package in our project by using npm like so:

npm install @testing-library/react --save-dev

Once we have added @testing-library/react to our project, we can import the two essential values, render and screen, into our tests.

render() is a function that we can use to virtually render components and make them available in our unit tests. Similar to ReactDOM.render(), RTL’s render() function takes in JSX as an argument.
screen is a special object which can be thought of as a representation of the browser window. We can make sure that our virtually rendered components are available in the test by using the screen.debug() method which prints out all the DOM contents.

The screen object has a few other useful methods that we’ll cover in the upcoming exercises but for now, let’s look at an example.

Look at the code snippet below, it shows the output of a unit test that prints out the DOM contents of the Greeting component.

import { render, screen } from '@testing-library/react'

const Greeting = () => {
  return (<h1>Hello World</h1>)
};

test('should prints out the contents of the DOM' () => {
    render(<Greeting />);
    screen.debug();
});

// Output:
<body>
  <div>
    <h1>
      Hello World
    </h1>
  </div>
</body>

After importing the render and screen values from '@testing-library/react', a test is created using the test() function from the Jest testing framework. Inside, the <Greeting> component is virtually rendered and then the resulting virtual DOM is printed via the screen.debug() method.

Notice how the output shows the rendered contents of <Greeting> (an <h1> element) and not the component itself. As was mentioned in the first exercise, React Testing Library strives to produce a testing environment that is as close to the user’s experience as possible.

Now, let’s get started with testing the Passing Thoughts application!

Instructions

Install @testing-library/react.

To verify that you have successfully added the package to your project, navigate to package.json and check that "@testing-library/react" appears in the "devDependencies" array.

In Thought.test.js import render() and screen from @testing-library/react.

Now, let’s try rendering the <Thought /> component in our test.

Inside the provided test() in Thought.test.js call the render() function and pass in the <Thought /> component. The <Thought /> component expects 2 props:

thought: use the provided thought object
removeThought: pass an empty function () => {}

Now, after the call to render(), call screen.debug() to see the rendered component.

In your terminal, run the npm test command to run the test. What do you see?

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