Testing stateful React hooks

Subject under test

import { useState } from 'react';

export function useCounter(initial = 0) {
  const [count, setCount] = useState(initial);

  return [count, () => setCount(count + 1)];
}

Unit test

import { useCounter } from './Calculator';

const mockSetState = jest.fn();

jest.mock('react', () => ({
  useState: initial => [initial, mockSetState]
}));

test('Can increment from 1 to 2', () => {
  const [_, increment] = useCounter(1);

  increment();

  expect(mockSetState).toHaveBeenCalledWith(2);
});

The following technique works well for me testing functional components with useState destructured:

import * as React from 'react';

describe('Some message', () => {
    const setState = jest.fn();
    // eslint-disable-next-line @typescript-eslint/no-explicit-any
    const useStateMock: any = (initState: any) => [initState, setState];

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

    it('Is a test where we want to mock useState', () => {
          jest.spyOn(React, 'useState').mockImplementation(useStateMock);
          const wrapper = shallow(<Component {...props} />);
          // trigger setState somehow
          expect(setState).toHaveBeenCalledTimes(1);
          // Other tests here
    });
});

This is typescript, but it should work just as well in JS if you remove the type annotations

@marco-souza how you solved using spyOn. You have any example? Thanks

@marco-souza how you solved using spyOn. You have any example? Thanks

import React, { useState as useStateMock } from 'react';

jest.mock('react', () => ({
  ...jest.requireActual('react'),
  useState: jest.fn(),
}));

describe('testing useState mocked', () => {
  const setState = jest.fn();
  const useStateMock = (initState: any) => [initState, setState];

  jest.spyOn(React, 'useState').mockImplementation(useStateMock);

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

  // your tests goes here
});

For me it's better having this logic in a custom hook and using this library for the tests instead https://github.com/testing-library/react-hooks-testing-library

how to spy specific useState hook, when I have more than one useState hook inside my component?

@muhsalaa, suppose you have a component like this:

const Component = () => {
    const setState1 = useState(1);
    const setState2 = useState(2);
    ...
};

In the test, you'll want to do:

import * as React from 'react';
...
it('Should work', () => {
    const useStateSpy = jest.spyOn(React, 'useState')
    ...
    expect(useStateSpy).toHaveBeenNthCalledWith(2, 2); // if testing the second `useState`
});

not working ...

How about
setState(prevState => ({ ...prevState, }))
?

How about
setState(prevState => ({ ...prevState, }))
?

Yeah What about class component How do I Change class setState ?

how mock many state in useState?

not working

 jest.spyOn(React, 'useState')
    .mockImplementationOnce(() => [false, () => null])
    .mockImplementationOnce(() => [true, () => null]) 

You may consider using renderHook (testing-library/react). I find it easy to use.

Here is the sample code:

const { result } = renderHook(() => {
      const [isOpen, setIsOpen] = useState(true)
      return { isOpen, setIsOpen }
    })

    const renderValue = await act(() => {
      return render(
        <ProductFormDialog
          isOpen={result.current.isOpen}
          setIsOpen={result.current.setIsOpen}
        />
      )
    })

    const displayValue = renderValue.getByTestId('header-container').textContent
    expect(displayValue).toEqual('Header')