setup function returns undefined when testing a custom hook usePrevious

Question

I have got code for usePrevious hook from somewhere on internet. The code for usePrevious looks like:

export const usePrevious = (value) => {
  const ref = useRef();
  useEffect(() => {
    ref.current = value;
  }, [value]);
  return ref.current;
};

Now, I am learing testing react with jest and enzyme. So, I tried to test usePrevious and got some problems. Here is my test case:

import React from 'react';
import { render } from 'enzyme';

import { usePrevious } from './customHooks';

const Component = ({ children, value }) => children(usePrevious(value));
const setup = (value) => {
  let returnVal = '';
  render(
    <Component value={value}>
      {
        (val) => {
          returnVal = val;
          return null;
        }
      }
    </Component>,
  );
  return returnVal;
};

describe('usePrevious', () => {
  it('returns something', () => {
    const test1 = setup('test');
    const test2 = setup(test1);
    expect(test2).toBe('test');
  });
});

When the test execution completes, I get this error:

Expected: 'test', Received: undefined

Can anyone please let me know why am I getting undefined and is this the correct way to test custom hoooks in react?

After suggestion from comments from @Dmitrii G, I have changed my code to re-render the component (Previously I was re-mounting the component).

Here is the updated code:

import React from 'react';
import PropTypes from 'prop-types';
import { shallow } from 'enzyme';

import { usePrevious } from './customHooks';

const Component = ({ value }) => {
  const hookResult = usePrevious(value);
  return (
    <div>
      <span>{hookResult}</span>
      <span>{value}</span>
    </div>
  );
};

Component.propTypes = {
  value: PropTypes.string,
};

Component.defaultProps = {
  value: '',
};


describe('usePrevious', () => {
  it('returns something', () => {
    const wrapper = shallow(<Component value="test" />);
    console.log('>>>>> first time', wrapper.find('div').childAt(1).text());
    expect(wrapper.find('div').childAt(0).text()).toBe('');

    // Test second render and effect
    wrapper.setProps({ value: 'test2' });
    console.log('>>>>> second time', wrapper.find('div').childAt(1).text());
    expect(wrapper.find('div').childAt(0).text()).toBe('test');
  });
});

But still I am getting the same error

Expected: "test", Received: ""

Tests Passes when settimeout is introduced:

import React from 'react';
import PropTypes from 'prop-types';
import { shallow } from 'enzyme';

import { usePrevious } from './customHooks';

const Component = ({ value }) => {
  const hookResult = usePrevious(value);
  return <span>{hookResult}</span>;
};

Component.propTypes = {
  value: PropTypes.string,
};

Component.defaultProps = {
  value: '',
};


describe('usePrevious', () => {
  it('returns empty string when component is rendered first time', () => {
    const wrapper = shallow(<Component value="test" />);
    setTimeout(() => {
      expect(wrapper.find('span').text()).toBe('');
    }, 0);
  });
  it('returns previous value when component is re-rendered', () => {
    const wrapper = shallow(<Component value="test" />);
    wrapper.setProps({ value: 'test2' });
    setTimeout(() => {
      expect(wrapper.find('span').text()).toBe('test');
    }, 0);
  });
});

I am not a big fan of using settimeout, so I feel that probably i am doing some mistake. If anyone knows a solution that does not use settimeout, feel free to post here. Thank you.

Answer 1

Enzyme by the hood utilizes React's shallow renderer. And it has issue with running effects . Not sure if it's going to be fixed soon. Workaround with setTimeout was a surprise to me, did not know it works. Unfortunately, it is not universal approach since you'd need to use that on any change that cases re-render. Really fragile.

As a solution you can use mount() instead.

Also you may mimic shallow rendering with mount() with mocking every nested component:

jest.mock("../MySomeComponent.jsx", () =>
  (props) => <span {...props}></span>
);