how to test onError behavior?

[teobler]

I have a prod code like this:

const myHook = () => {
  const [error, setError] = useState("");
  const { data } = useSWR(url, fetcher, {
    onError: (error) => {
      setError(error);
    },
  });

  return { data, error, setError };
};

then I want to mock this error and perform an assertion for error message changing:

import { renderHook } from "@testing-library/react-hooks";

describe("#myHook", () => {
  it("should set error message when request failed", async () => {
    // given
    const useSWR = jest.fn().mockReturnValue(Error("error"));
    jest.doMock("swr", () => useSWR);
    const { myHook } = require("../myHook");

    // when
    const { result, waitForNextUpdate } = renderHook(() => myHook());

    await waitForNextUpdate;

    // then
    expect(result.current.error).toBe("error");
  });
});

and it does not work, so my question is how to mock useSWR's error, so I can trigger onError callback

Thanks.

[sergiodxa]

Your fetcher function should throw an error, not the hook itself, also you can access the error with

const { data, error } = useSWR(key, fetcher)

You don’t need to create a different state and use onError to update it.

[teobler]

Thanks for your quick reply, you are right, I make the wrong way for testing.

And thanks for your advice~