How to set initial state for useState Hook in jest and enzyme?

Currently Im using functional component with react hooks. But I'm unable to test the useState hook completely. Consider a scenario like, in useEffect hook I'm doing an API call and setting value in the useState. For jest/enzyme I have mocked data to test but I'm unable to set initial state value for useState in jest.

const [state, setState] = useState([]);

I want to set initial state as array of object in jest. I could not find any setState function as similar like class component.

You can mock React.useState to return a different initial state in your tests:

// Cache original functionality
const realUseState = React.useState

// Stub the initial state
const stubInitialState = ['stub data']

// Mock useState before rendering your component
jest
  .spyOn(React, 'useState')
  .mockImplementationOnce(() => realUseState(stubInitialState))

Reference: https://dev.to/theactualgivens/testing-react-hook-state-changes-2oga

First, you cannot use destructuring in your component. For example, you cannot use:

import React, { useState } from 'react';
const [myState, setMyState] = useState();

Instead, you have to use:

import React from 'react'
const [myState, setMyState] = React.useState();

Then in your test.js file:

test('useState mock', () => {
   const myInitialState = 'My Initial State'

   React.useState = jest.fn().mockReturnValue([myInitialState, {}])
   
   const wrapper = shallow(<MyComponent />)

   // initial state is set and you can now test your component 
}

If you use useState hook multiple times in your component:

// in MyComponent.js

import React from 'react'

const [myFirstState, setMyFirstState] = React.useState();
const [mySecondState, setMySecondState] = React.useState();

// in MyComponent.test.js

test('useState mock', () => {
   const initialStateForFirstUseStateCall = 'My First Initial State'
   const initialStateForSecondUseStateCall = 'My Second Initial State'

   React.useState = jest.fn()
     .mockReturnValueOnce([initialStateForFirstUseStateCall, {}])
     .mockReturnValueOnce([initialStateForSecondUseStateCall, {}])
   
   const wrapper = shallow(<MyComponent />)

   // initial states are set and you can now test your component 
}
// actually testing of many `useEffect` calls sequentially as shown
// above makes your test fragile. I would recommend to use 
// `useReducer` instead.

If I recall correctly, you should try to avoid mocking out the built-in hooks like useState and useEffect. If it is difficult to trigger the state change using enzyme's invoke(), then that may be an indicator that your component would benefit from being broken up.

• Below function will return state

const setHookState = (newState) => jest.fn().mockImplementation(() => [
  newState,
  () => {},
]);

• Add below to use react

const reactMock = require('react');

In your code, you must use React.useState() to this work, else it won't work

const [arrayValues, setArrayValues] = React.useState();

const [isFetching, setFetching] = React.useState();

• Then in your test add following, mock state values

reactMock.useState = setHookState({ arrayValues: [], isFetching: false, });

Inspiration: Goto

//Component    
const MyComponent = ({ someColl, someId }) => {
     const [myState, setMyState] = useState(null);

     useEffect(() => {loop every time group is set
         if (groupId) {
             const runEffect = async () => {
                  const data = someColl.find(s => s.id = someId);
                  setMyState(data);
             };
             runEffect();
         }
     }, [someId, someColl]);

     return (<div>{myState.name}</div>);
};

// Test
// Mock
const mockSetState = jest.fn();
jest.mock('react', () => ({
    ...jest.requireActual('react'),
    useState: initial => [initial, mockSetState]
}));
const coll = [{id: 1, name:'Test'}, {id: 2, name:'Test2'}];

it('renders correctly with groupId', () => {
    const wrapper = shallow(
        <MyComponent comeId={1} someColl={coll} />
    );
    setTimeout(() => {
        expect(wrapper).toMatchSnapshot();
        expect(mockSetState).toHaveBeenCalledWith({ id: 1, name: 'Test' });
    }, 100);
});