React Note: Shallow Rendering Test

Last week, we talked about the most basic method for testing a React element, which is smoke testing. Today, we'll talk about a slightly more advanced method, namely shallow rendering test. We will use Jest and Enzyme for this purpose.

Shallow Rendering Test

The main purpose of shallow rendering test is to test a single component isolated from its child components. So you can focus on testing the component without worrying about its children.

To do a shallow rendering test, follow these instructions:

  1. Start by installing Enzyme and its accompanying packages:
    npm install --save enzyme enzyme-adapter-react-16 react-test-renderer
  2. You'll need to configure Enzyme first. To do so, create a new file called setupTests.js in the src folder of your project. Here's the content of that file:

    // src/setupTests.js
    
    import { configure } from "enzyme";
    import Adapter from "enzyme-adapter-react-16";
    
    configure({adapter: new Adapter()});

  3. We will test a component called MyCard, as seen below, which contains a counter and a button to increment the counter. You can also pass a children component to it.
    // src/MyCard.js
    
    import React from "react";
    
    const MyCard = ({counter, children}) => {
      const [count, setCount] = React.useState(counter);
    
      const handleClick = () => {
        setCount(count+1);
      }
    
      return (
        <div>
          <p>Counter: {count}</p>
          <button onClick={handleClick}>Increment</button>
          <div>
            {children}
          </div>
        </div>
      );
    }
    
    export default MyCard;

  4. Create a new file called MyCard.test.js in the same folder as MyCard.js with the following content:
    // src/MyCard.test.js
    
    import React from "react";
    import MyCard from "./MyCard";
    import { shallow } from "enzyme";
    
    describe("<MyCard />", () => {
    
      // Test #1
      it("renders without crashing", () => {
        shallow(<MyCard />);
      });
    
      // Test #2
      it("renders counter correctly", () => {
        const wrapper = shallow(
          <MyCard counter={0} />
        );
        expect(wrapper.find("p").text()).toEqual("Counter: 0");
      });
    
      // Test #3
      it("renders children when passed in", () => {
        const wrapper = shallow((
          <MyCard>
            <p>Hi I am a child component</p>
          </MyCard>
        ));
        expect(wrapper.contains(<p>Hi I am a child component</p>)).toEqual(true);
      });
    
      // Test #4
      it("simulates click events", () => {
        const wrapper = shallow((
          <MyCard 
            counter={0}
          />
        ));
        wrapper.find("button").simulate("click");
        expect(wrapper.find("p").text()).toEqual("Counter: 1");
      });
    });
    There are four separate tests here.
    The first one is quite similar to smoke testing. The second and third tests make sure that the rendering works as intended. And finally, the fourth test simulates a clicking event of the increment button.
  5. To run the tests, simply run the following command in the project's root folder:
    $ npm test
    // or
    $ npm test -- --coverage‚Äč

  

That's it for shallow rendering test!