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Test-driven Development with React

Manipulating State of Stateful Component in Tests

Let’s learn how to manipulate states of a stateful component through Enzyme's method in tests.

We'll cover the following...

Manipulating state in tests

So far, the carousel is just a pair of buttons. What it really needs is some slides. A simple approach to this is to give the Carousel instance an array of data as a prop called slides, then pass the data from slides[slideIndex] as props to a CarouselSlide.

  • Replace the existing beforeEach block to set the slides prop on the Carousel instance, then add a (failing) test for the slide-passing behavior:
Javascript (babel-node)
import React from 'react';
import { shallow } from 'enzyme';
import Carousel from '../Carousel';
import CarouselButton from '../CarouselButton';
import CarouselSlide from '../CarouselSlide';
describe('Carousel', () => {
  let wrapper;
  const slides = [
    {
      imgUrl: 'https://example.com/slide1.png',
      description: 'Slide 1',
      attribution: 'Uno Pizzeria',
    },
    {
      imgUrl: 'https://example.com/slide2.png',
      description: 'Slide 2',
      attribution: 'Dos Equis',
    },
    {
      imgUrl: 'https://example.com/slide3.png',
      description: 'Slide 3',
      attribution: 'Three Amigos',
    },
  ];
  beforeEach(() => {
    wrapper = shallow(<Carousel slides={slides} />);
  });
  
  ...
  it('renders the current slide as a CarouselSlide', () => {
    let slideProps;
    slideProps = wrapper.find(CarouselSlide).props(); // 1
    expect(slideProps).toEqual(slides[0]); // 2
    wrapper.setState({ slideIndex: 1 }); // 3
    slideProps = wrapper.find(CarouselSlide).props();
    expect(slideProps).toEqual(slides[1]);
  });
});
  1. The new test uses a method called props() instead of prop(). This method returns an object containing all of the
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