Testing React components that fetches data using Hooks

Question

My React-application has a component that fetches data to display from a remote server. In the pre-hooks era, componentDidMount() was the place to go. But now I wanted to use hooks for this.

const App = () => {
  const [ state, setState ] = useState(0);
  useEffect(() => {
    fetchData().then(setState);
  });
  return (
    <div>... data display ...</div>
  );
};

And my test using Jest and Enzyme looks like this:

import React from 'react';
import { mount } from 'enzyme';
import App from './App';
import { act } from 'react-test-renderer';

jest.mock('./api');

import { fetchData } from './api';

describe('<App />', () => {
  it('renders without crashing', (done) => {
    fetchData.mockImplementation(() => {
      return Promise.resolve(42);
    });
    act(() => mount(<App />));
    setTimeout(() => {
      // expectations here
      done();
    }, 500);
  });  
});

The test succeeds, but it logs a few warnings:

console.error node_modules/react-dom/cjs/react-dom.development.js:506
    Warning: An update to App inside a test was not wrapped in act(...).

    When testing, code that causes React state updates should be wrapped into act(...):

    act(() => {
    /* fire events that update state */
    });
    /* assert on the output */

    This ensures that you're testing the behavior the user would see in the browser. Learn more at (redacted)
        in App (created by WrapperComponent)
        in WrapperComponent

The only update to the App component happens from the Promise callback. How can I ensure this happens within the act block? The docs clearly suggest to have assertions happen outside the act block. Besides, putting them inside doesn't change the warning.

Answer 1

That issue is caused by many updates inside Component.

I got the same issue, this would solve the issue.

await act( async () => mount(<App />));

Answer 2

I resolved this issue using below steps

1. Update react and react-dom to 16.9.0 version.
2. Install regenerator-runtime

3. import regenerator-runtime in setup file.

   import "regenerator-runtime/runtime";
   import { configure } from "enzyme";
   import Adapter from "enzyme-adapter-react-16";
   
   configure({
    adapter: new Adapter()
   });
   

4. Wrap mount and other possible actions which are going to cause state changes inside act. import act from simple react-dom/test-utils, async & await like below.

   import React from 'react';
   import { mount } from 'enzyme';
   import App from './App';
   import { act } from "react-dom/test-utils";
   
   jest.mock('./api');
   
   import { fetchData } from './api';
   
   describe('<App />', () => {
   it('renders without crashing',  async (done) => {
     fetchData.mockImplementation(() => {
       return Promise.resolve(42);
     });
     await act(() => mount(<App />));
     setTimeout(() => {
       // expectations here
       done();
     }, 500);
    });  
   });
   

Hope this helps.

Answer 3

I have created examples for testing async hooks.

https://github.com/oshri6688/react-async-hooks-testing

CommentWithHooks.js:

import { getData } from "services/dataService";

const CommentWithHooks = () => {
  const [data, setData] = useState(null);
  const [isLoading, setIsLoading] = useState(true);

  const fetchData = () => {
    setIsLoading(true);

    getData()
      .then(data => {
        setData(data);
      })
      .catch(err => {
        setData("No Data");
      })
      .finally(() => {
        setIsLoading(false);
      });
  };

  useEffect(() => {
    fetchData();
  }, []);

  return (
    <div>
      {isLoading ? (
        <span data-test-id="loading">Loading...</span>
      ) : (
        <span data-test-id="data">{data}</span>
      )}

      <button
        style={{ marginLeft: "20px" }}
        data-test-id="btn-refetch"
        onClick={fetchData}
      >
        refetch data
      </button>
    </div>
  );
};

CommentWithHooks.test.js:

import React from "react";
import { mount } from "enzyme";
import { act } from "react-dom/test-utils";
import MockPromise from "testUtils/MockPromise";
import CommentWithHooks from "./CommentWithHooks";
import { getData } from "services/dataService";

jest.mock("services/dataService", () => ({
  getData: jest.fn(),
}));

let getDataPromise;

getData.mockImplementation(() => {
  getDataPromise = new MockPromise();

  return getDataPromise;
});

describe("CommentWithHooks", () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  it("when fetching data successed", async () => {
    const wrapper = mount(<CommentWithHooks />);
    const button = wrapper.find('[data-test-id="btn-refetch"]');
    let loadingNode = wrapper.find('[data-test-id="loading"]');
    let dataNode = wrapper.find('[data-test-id="data"]');

    const data = "test Data";

    expect(loadingNode).toHaveLength(1);
    expect(loadingNode.text()).toBe("Loading...");

    expect(dataNode).toHaveLength(0);

    expect(button).toHaveLength(1);
    expect(button.prop("onClick")).toBeInstanceOf(Function);

    await getDataPromise.resolve(data);

    wrapper.update();

    loadingNode = wrapper.find('[data-test-id="loading"]');
    dataNode = wrapper.find('[data-test-id="data"]');

    expect(loadingNode).toHaveLength(0);

    expect(dataNode).toHaveLength(1);
    expect(dataNode.text()).toBe(data);
  });

testUtils/MockPromise.js:

import { act } from "react-dom/test-utils";

const createMockCallback = callback => (...args) => {
  let result;

  if (!callback) {
    return;
  }

  act(() => {
    result = callback(...args);
  });

  return result;
};

export default class MockPromise {
  constructor() {
    this.promise = new Promise((resolve, reject) => {
      this.promiseResolve = resolve;
      this.promiseReject = reject;
    });
  }

  resolve(...args) {
    this.promiseResolve(...args);

    return this;
  }

  reject(...args) {
    this.promiseReject(...args);

    return this;
  }

  then(...callbacks) {
    const mockCallbacks = callbacks.map(callback =>
      createMockCallback(callback)
    );

    this.promise = this.promise.then(...mockCallbacks);

    return this;
  }

  catch(callback) {
    const mockCallback = createMockCallback(callback);

    this.promise = this.promise.catch(mockCallback);

    return this;
  }

  finally(callback) {
    const mockCallback = createMockCallback(callback);

    this.promise = this.promise.finally(mockCallback);

    return this;
  }
}

Answer 4

Enzyme doesn't have support for hooks since it's a relatively new feature: https://github.com/airbnb/enzyme/issues/2011

Maybe you can use plain Jest in the meantime? Also don't worry about the warning, it's supposed to go away when React 16.9.0 is released (see this pull request https://github.com/facebook/react/pull/14853)

Answer 5

I had that exact same problem, and ended up writing a library that solves this issue by mocking all the standards React Hooks.

Basically, act() is a synchronous function, like useEffect, but useEffect executes an async function. There's no way that act() would be able to "wait" for that to execute. Fire and forget!

Article here: https://medium.com/@jantoine/another-take-on-testing-custom-react-hooks-4461458935d4

Library here: https://github.com/antoinejaussoin/jooks

To test your code, you would first need to extract your logic (the fetch, etc.) into a separate custom hook: something like:

const useFetchData = () => {
  const [ state, setState ] = useState(0);
  useEffect(() => {     
    fetchData().then(setState);
  });
  return state;
}

Then, using Jooks, your test would look like:

import init from 'jooks';
[...]
describe('Testing my hook', () => {
  const jooks = init(() => useFetchData());

  // Mock your API call here, by returning 'some mocked value';

  it('Should first return 0', () => {
    const data = jooks.run();
    expect(data).toBe(0);
  });

  it('Then should fetch the data and return it', async () => {
    await jooks.mount(); // Fire useEffect etc.
    const data = jooks.run();
    expect(data).toBe('some mocked value');
  });
});