jest.setup.js

import { configure } from 'enzyme';
import Adapter from 'enzyme-adapter-react-16';
import '@testing-library/jest-dom';

configure({ adapter: new Adapter() });

const observerMap = new Map();
const instanceMap = new Map();

beforeAll(() => {
    // Stub `window.scroll` since Jest will fail otherwise. This may change in the future.
    global.scroll = () => true;

    // Some tests are run in the Jest `node` environment, where `window` is not defined, and these
    // mocks are not needed.
    if (typeof window !== 'undefined') {
        // Mock `window.IntersectionObserver` using code from these two places:
        // https://github.com/thebuilder/react-intersection-observer/blob/e31086c713615f3cfbe60eaa13491adcee3d41c2/src/test-utils.ts#L83
        // https://jestjs.io/docs/en/manual-mocks#mocking-methods-which-are-not-implemented-in-jsdom
        Object.defineProperty(window, 'IntersectionObserver', {
            writable: true,
            value: jest.fn((cb, options) => {
                const instance = {
                    thresholds: Array.isArray(options.threshold)
                        ? options.threshold
                        : [options.threshold],
                    root: options.root,
                    rootMargin: options.rootMargin,
                    time: Date.now(),
                    observe: jest.fn(element => {
                        instanceMap.set(element, instance);
                        observerMap.set(element, cb);
                    }),
                    unobserve: jest.fn(element => {
                        instanceMap.delete(element);
                        observerMap.delete(element);
                    }),
                    disconnect: jest.fn(),
                };
                return instance;
            }),
        });
    }
});

afterEach(() => {
    // Some tests are run in the Jest `node` environment, where `window` is not defined, and these
    // mocks are not needed.
    if (typeof window !== 'undefined') {
        window.IntersectionObserver.mockClear();
        instanceMap.clear();
        observerMap.clear();
    }
});