React useReducer

The useReducer Hook is similar to the useState Hook.

It allows for custom state logic.

If you find yourself keeping track of multiple pieces of state that rely on complex logic, useReducer may be useful.

Syntax

The useReducer Hook accepts three arguments.

useReducer(reducer, initialState, init)

The reducer function contains your custom state logic and the initialStatecan be a simple value, but generally will contain an object. The init argument is optional and is used to initialize the state.

The useReducer Hook returns the current stateand a dispatchmethod.

Here is an example where we use useReducer to keep track of the score of two players:

Example:

import { useReducer } from 'react';
import { createRoot } from 'react-dom/client';

const initialScore = [
  {
    id: 1,
    score: 0,
    name: "John",
  },
  {
    id: 2,
    score: 0,
    name: "Sally",
  },
];

const reducer = (state, action) => {
  switch (action.type) {
    case "INCREASE":
      return state.map((player) => {
        if (player.id === action.id) {
          return { ...player, score: player.score + 1 };
        } else {
          return player;
        }
      });
    default:
      return state;
  }
};

function Score() {
  const [score, dispatch] = useReducer(reducer, initialScore);

  const handleIncrease = (player) => {
    dispatch({ type: "INCREASE", id: player.id });
  };

  return (
    <>
      {score.map((player) => (
        <div key={player.id}>
          <label>
            <input
              type="button"
              onClick={() => handleIncrease(player)}
              value={player.name}
            />
            {player.score}
          </label>
        </div>
      ))}
    </>
  );
}

createRoot(document.getElementById('root')).render(
  <Score />
);