useReducer hook: managing complex state logic

useReducer for Complex State

useReducer is useful when state transitions involve multiple values or complex logic. It centralizes state update logic into a single reducer function, making it predictable and testable.

Syntax

import { useReducer } from 'react';

const initialState = { count: 0, step: 1 };

function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return { ...state, count: state.count + state.step };
    case 'decrement':
      return { ...state, count: state.count - state.step };
    case 'setStep':
      return { ...state, step: action.payload };
    case 'reset':
      return initialState;
    default:
      throw new Error('Unknown action: ' + action.type);
  }
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);

  return (
    <div>
      <p>Count: {state.count} | Step: {state.step}</p>
      <button onClick={() => dispatch({ type: 'increment' })}>+</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>-</button>
      <input
        type="number"
        value={state.step}
        onChange={e => dispatch({ type: 'setStep', payload: Number(e.target.value) })}
      />
      <button onClick={() => dispatch({ type: 'reset' })}>Reset</button>
    </div>
  );
}

Prefer useReducer over useState when: you have 3+ related state values, next state depends on previous in non-trivial ways, or you want testable state logic. The reducer is a pure function — easy to unit test in isolation.