ReactJS - prevState in the new useState React hook?

Question

I really like the new React hooks and I\'m using them frequently for a project I\'m working on. I\'m coming across a situation where I want to use the prevState in the

Answer 1

For objects you can use the spread operator to use prevState within your setState call.

const [stateObject, setObjectState] = useState({
  firstKey: '',
  secondKey: '',
});

setObjectState((prevState) => ({
  ...prevState,
  secondKey: 'value',
}));

// stateObject = {
//   firstKey: '',
//   secondKey: 'value',
// }

The snippet below show an example of using prevState for setting the state of an object.

const {useState} = React;

const Example = ({title}) => {
  const initialState = {
    firstKey: 'empty',
    secondKey: 'empty',
    thirdKey: 'not empty',
  }
  const [object, setObject] = useState(initialState);
  
  const withPrevState = () => {
    setObject((prevState) => ({
      ...prevState,
      secondKey: 'not empty',
    }));
  }

  return (
    <div>
      <h5>Updates Second key to 'not empty'</h5>
      <p>First key: {object.firstKey}</p>
      <p>Second key: {object.secondKey}</p>
      <p>Third key: {object.thirdKey}</p>
      <button onClick={withPrevState}>
        Update with prevState
      </button>
      <button onClick={() => {setObject({secondKey: 'not empty'})}}>
        Update without prevState
      </button>
      <button onClick={() => {setObject(initialState)}}>
        Reset
      </button>
    </div>
  );
};

// Render it
ReactDOM.render(
  <Example />,
  document.getElementById("react")
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id="react"></div>

Answer 2

You have already the previous state in the destructed variable: someState

so you can do:

const [ someState, setSomeState ] = useState( new Map() )
setSomeState( someState.set( key, value ) )

Answer 3

In order to use Map objects, you'll need to clone it before manipulating the values. Otherwise, it's mutating the original Map object and React doesn't handle mutatable state.

const handleChange = useCallback(({ target: { name, checked } }) => {
  setCheckbox(prevState => {
    return new Map(prevState).set(name, checked);
  });
}, []);

Updated Working Example:

Answer 4

state updater from useState provides a callback pattern which returns you the previous state which you can use to update the current state

const [ someState, setSomeState ] = useState( new Map() )
setSomeState(prevState => prevState.set( key, value ) )