React Unit Testing Example 13

LoadingIndicator.test.js

  describe('when isLoading is true', () => {
    describe('given 200ms have elapsed', () => {
      it('should render loading indicator', () => {
        jest.useFakeTimers();
        const wrapper = mount(
          <LoadingIndicator isLoading={true}>
            <div>ahoy!</div>
          </LoadingIndicator>
        );

        // assert that setTimeout was called exactly once
        expect(setTimeout.mock.calls.length).toEqual(1);

        // assert that the 2nd argument to the call to setTimeout is 200
        expect(setTimeout.mock.calls[0][1]).toEqual(200);

        jest.runAllTimers();

        expect(wrapper.html()).toBe('<div>loading...</div>');
        wrapper.unmount();
      });
    });
  });