[Solved]-How do I test component methods on a React component that are defined as arrow functions (class properties)?-Reactjs

Even if the rendering is shallow, you can call the wrapper.instance() method.

it("should call sort ids", () => {
    const wrapper = shallow(<Chat />);
    wrapper.instance().sortIds = jest.fn();
    wrapper.update();    // Force re-rendering 
    wrapper.instance().componentDidMount();
    expect(wrapper.instance().sortIds).toBeCalled();
 });