component-testing.md

Component testing

Unit testing your Redux actions and reducers is nice, but you can do even more to make sure nothing breaks your application. Since React is the view layer of your app, let's see how to test Components too!

• Shallow rendering
• Enzyme

Shallow rendering

React provides us with a nice add-on called the Shallow Renderer. This renderer will render a React component one level deep. Lets take a look at what that means with a simple <Button> component...

This component renders a <button> element containing a checkmark icon and some text:

// Button.react.js

import CheckmarkIcon from './CheckmarkIcon.react';

function Button(props) {
  return (
    <button className="btn" onClick={props.onClick}>
      <CheckmarkIcon />
      { React.Children.only(props.children) }
    </button>
  );
}

export default Button;

Note: This is a stateless ("dumb") component

It might be used in another component like this:

// HomePage.react.js

import Button from './Button.react';

class HomePage extends React.Component {
  render() {
    return(
      <Button onClick={this.doSomething}>Click me!</Button>
    );
  }
}

Note: This is a stateful ("smart") component!

When rendered normally with the standard ReactDOM.render function, this will be the HTML output (Comments added in parallel to compare structures in HTML from JSX source):

<button>                           <!-- <Button>             -->
  <i class="fa fa-checkmark"></i>  <!--   <CheckmarkIcon />  -->
  Click Me!                        <!--   { props.children } -->
</button>                          <!-- </Button>            -->

Conversely, when rendered with the shallow renderer, we'll get a String containing this "HTML":

<button>              <!-- <Button>             -->
  <CheckmarkIcon />   <!--   NOT RENDERED!      -->
  Click Me!           <!--   { props.children } -->
</button>             <!-- </Button>            -->

If we test our Button with the normal renderer and there's a problem with the CheckmarkIcon then the test for the Button will fail as well... but finding the culprit will be hard. Using the shallow renderer, we isolate the problem's cause since we don't render any other components other than the one we're testing!

The problem with the shallow renderer is that all assertions have to be done manually, and you cannot do anything that needs the DOM.

Thankfully, AirBnB has open sourced their wrapper around the React shallow renderer and jsdom, called enzyme. enzyme is a testing utility that gives us a nice assertion/traversal/manipulation API.

Enzyme

Lets test our <Button> component! We're going to assess three things: First, that it renders a HTML <button> tag, second that it renders its children we pass it and third that handles clicks!

This is our Jest setup:

describe('<Button />', () => {
  it('renders a <button>', () => {});

  it('renders its children', () => {});

  it('handles clicks', () => {});
});

Lets start with testing that it renders a <button>. To do that we first shallow render it, and then expect that a <button> node exists.

it('renders a <button>', () => {
  const renderedComponent = shallow(
    <Button></Button>
  );
  expect(
    renderedComponent.find("button").node
  ).toBeDefined();
});

Nice! If somebody breaks our button component by having it render an <a> tag or something else we'll immediately know! Let's do something a bit more advanced now, and check that our <Button> renders its children.

We render our button component with some text, and then verify that our text exists:

it('renders its children', () => {
  const text = "Click me!";
  const renderedComponent = shallow(
    <Button>{ text }</Button>
  );
  expect(
    renderedComponent.contains(text)
  ).toEqual(true);
});

Great! Onwards to our last and most advanced test: checking that our <Button> handles clicks correctly. We'll use a Spy for that. A Spy is a function that knows if, and how often, it has been called. We create the Spy (thoughtfully provided by expect), pass it as the onClick handler to our component, simulate a click on the rendered <button> element and, lastly, see that our Spy was called:

it('handles clicks', () => {
  const onClickSpy = jest.fn();
  const renderedComponent = shallow(<Button onClick={onClickSpy} />);
  renderedComponent.find('button').simulate('click');
  expect(onClickSpy).toHaveBeenCalled();
});

And that's how you unit test your components and make sure they work correctly!