How can I test a React Hooks component by changing useState

Question

I have a React hooks functional component that I'd like to test with Jest/Enzyme. I would like test its tertiary render behaviour based upon a useState value. I can't seem to find any example online. There is no 'click' to simulate - no API call to mock because at the end, I still need to test based upon the useState value. In the past, with class components, I could set the state. With the new hooks, I can't. So, basically - how do I mock an async await inside a mocked submitForm function so that the render behaves properly?

Here's my component:

import React, { useState } from 'react';
import { Redirect } from 'react-router-dom';

import Form from 'core/Form';

export const Parent = ({submitForm}) => {
  const [formValues, setFormValues] = useState({});
  const [redirect, setRedirect] = useState(false);

  const handleChange = name => evt => {
    setFormValues({ ...formValues, [name]: evt.target.value });
  };

  const onSubmit = async () => {
      try {
        const res = await submitForm(formValues);
        if (res) setRedirect(true);
        else setRedirect(false);
      } catch (err) {
        console.log('Submit error: ', err);
      }
  };

  return redirect ? (
    <Redirect push to={path} />
  ) : (
    <Form onSubmit={onSubmit} values={formValues} onChange={handleChange} />
  );
};

export default Parent;

Here's my testing so far:

import React from 'react';
import { shallow } from 'enzyme';
import { Redirect } from 'react-router-dom';

import Parent from './Parent';
import Form from 'core/Form';

let wrapper, props;
.
.
.

describe('<Parent /> rendering', () => {
  beforeEach(() => {
    props = createTestProps();
    wrapper = shallow(<Parent {...props} />);
  });

  afterEach(() => {
    jest.clearAllMocks();
  });

  const setState = jest.fn();
  const useStateSpy = jest.spyOn(React, 'useState');
  useStateSpy.mockImplementation(init => [init, setState]);

  it('Should render 1 Form', () => {
    expect(wrapper.find(Form)).toHaveLength(1);
  });

  it('renders Redirect after API call', () => {
    setRedirect = jest.fn(() => false);

    expect(wrapper.find(Redirect)).toHaveLength(1);
  });

  it('renders Form before API call', () => {
    setRedirect = jest.fn(() => true);

    expect(wrapper.find(Form)).toHaveLength(1);
  });
});

Answer 1

You don't need to spy useState hook. Which means you should not test these hooks and methods of the component directly. Instead, you should test components' behavior(the state, props and what is rendered)

E.g.

index.tsx:

import React, { useState } from 'react';
import { Redirect } from 'react-router-dom';

export const Form = ({ onSubmit, onChange, values }) => <form onSubmit={onSubmit}></form>;
const path = '/user';

export const Parent = ({ submitForm }) => {
  const [formValues, setFormValues] = useState({});
  const [redirect, setRedirect] = useState(false);

  const handleChange = (name) => (evt) => {
    setFormValues({ ...formValues, [name]: evt.target.value });
  };

  const onSubmit = async () => {
    try {
      const res = await submitForm(formValues);
      if (res) setRedirect(true);
      else setRedirect(false);
    } catch (err) {
      console.log('Submit error: ', err);
    }
  };

  return redirect ? (
    <Redirect push to={path} />
  ) : (
    <Form onSubmit={onSubmit} values={formValues} onChange={handleChange} />
  );
};

export default Parent;

index.test.tsx:

import Parent, { Form } from './';
import React from 'react';
import { shallow } from 'enzyme';
import { Redirect } from 'react-router-dom';
import { act } from 'react-dom/test-utils';

const whenStable = async () =>
  await act(async () => {
    await new Promise((resolve) => setTimeout(resolve, 0));
  });

describe('60137762', () => {
  it('should render Form', () => {
    const props = { submitForm: jest.fn() };
    const wrapper = shallow(<Parent {...props}></Parent>);
    expect(wrapper.find(Form)).toBeTruthy();
  });

  it('should handle submit and render Redirect', async () => {
    const props = { submitForm: jest.fn().mockResolvedValueOnce(true) };
    const wrapper = shallow(<Parent {...props}></Parent>);
    wrapper.find(Form).simulate('submit');
    await whenStable();
    expect(props.submitForm).toBeCalledWith({});
    expect(wrapper.find(Redirect)).toBeTruthy();
  });

  it('should handle submit and render Form', async () => {
    const props = { submitForm: jest.fn().mockResolvedValueOnce(false) };
    const wrapper = shallow(<Parent {...props}></Parent>);
    wrapper.find(Form).simulate('submit');
    await whenStable();
    expect(props.submitForm).toBeCalledWith({});
    expect(wrapper.find(Form)).toBeTruthy();
  });

  it('should handle error if submit failure', async () => {
    const logSpy = jest.spyOn(console, 'log');
    const mError = new Error('network');
    const props = { submitForm: jest.fn().mockRejectedValueOnce(mError) };
    const wrapper = shallow(<Parent {...props}></Parent>);
    wrapper.find(Form).simulate('submit');
    await whenStable();
    expect(props.submitForm).toBeCalledWith({});
    expect(logSpy).toHaveBeenCalledWith('Submit error: ', mError);
  });
});

Unit test results with coverage report:

 PASS  stackoverflow/60137762/index.test.tsx
  60137762
    ✓ should render Form (18ms)
    ✓ should handle submit and render Redirect (15ms)
    ✓ should handle submit and render Form (8ms)
    ✓ should handle error if submit failure (18ms)

  console.log node_modules/jest-environment-enzyme/node_modules/jest-mock/build/index.js:866
    Submit error:  Error: network
        at /Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/stackoverflow/60137762/index.test.tsx:39:20
        at step (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/stackoverflow/60137762/index.test.tsx:44:23)
        at Object.next (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/stackoverflow/60137762/index.test.tsx:25:53)
        at /Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/stackoverflow/60137762/index.test.tsx:19:71
        at new Promise (<anonymous>)
        at Object.<anonymous>.__awaiter (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/stackoverflow/60137762/index.test.tsx:15:12)
        at Object.<anonymous> (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/stackoverflow/60137762/index.test.tsx:37:47)
        at Object.asyncJestTest (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/jasmineAsyncInstall.js:100:37)
        at resolve (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/queueRunner.js:43:12)
        at new Promise (<anonymous>)
        at mapper (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/queueRunner.js:26:19)
        at promise.then (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/queueRunner.js:73:41)

-----------|---------|----------|---------|---------|-------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-----------|---------|----------|---------|---------|-------------------
All files  |   78.57 |      100 |      40 |   93.75 |                   
 index.tsx |   78.57 |      100 |      40 |   93.75 | 12                
-----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       4 passed, 4 total
Snapshots:   0 total
Time:        3.716s, estimated 5s

Source code: https://github.com/mrdulin/react-apollo-graphql-starter-kit/tree/master/stackoverflow/60137762