使用 React 测试库的 ReactJS 测试中的 Typescript 错误

Question

I'm using ReactJS and Material UI and Typescript. I want to test my menu - should show after I click on button with label Location . New menu should contain Location 1 item.

describe('<LocationsMenu />', () => {
    let component: RenderResult;

    beforeEach(() => {
        component = render(<LocationsMenu />);
    });

    it('should show and hide on click on top item trigger', async () => {
        const button = component.getByText('Locations').parentElement;
        await act(async () => {
            fireEvent.click(button);
        });
        expect(component.getByText('Location 1')).toBeDefined();
    });
});

This works - test passes.

Visual Studio Code shows me error in fireEvent.click(button) line: Argument of type 'HTMLElement | null' is not assignable to parameter of type 'Element | Node | Document | Window'. Argument of type 'HTMLElement | null' is not assignable to parameter of type 'Element | Node | Document | Window'. . How can I avoid it? I know I can do type casting like:

fireEvent.click(button as Element);

or

const button = component.getByText('Locations').parentElement as Element;

But maybe there is a better solution.

Answer 1

Typescript can deduce your variable type at any position in the source file, you need to 'cast' the button to HTMLElement instead of HTMLElement | null HTMLElement | null by checking if it's not null

// button may be null
const button = component.getByText('Locations').parentElement;

if (button) {
  // at this point, typescript knows that button cannot be null
  // so it has HTMLElement type in the block
  fireEvent.click(button);
}

Also note that I don't wrap fireEvent.click() inside act() . It's because react-testing-library has already done that for you so it's unnecessary here.

Answer 2

If what you want is brevity and you are absolutely sure the the element exists or the test would failed otherwise, you can ensure typescript by adding a non-null assertion operator like this

// button may be null, but you say it's not
const button = component.getByText('Locations').parentElement!;

fireEvent.click(button);