Using Reducers

useReducer Patterns

When to Use useReducer

Use useReducer when:

• Multiple related state values
• Complex state update logic
• Next state depends on previous state
• State transitions follow patterns

Use useState when:

• Simple independent values
• Basic toggle or counter logic
• No complex interdependencies

Basic Pattern

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 (
    <>
      <p>Count: {state.count}</p>
      <p>Step: {state.step}</p>

      <button => dispatch({ type: 'increment' })}>+</button>
      <button => dispatch({ type: 'decrement' })}>-</button>

      <input
        type="number"
        value={state.step}
        => dispatch({ type: 'setStep', payload: +e.target.value })}
      />

      <button => dispatch({ type: 'reset' })}>Reset</button>
    </>
  );
}

Complex Example: Todo List

const initialState = {
  todos: [],
  filter: 'all',
};

function todosReducer(state, action) {
  switch (action.type) {
    case 'added':
      return {
        ...state,
        todos: [...state.todos, { id: Date.now(), text: action.text, done: false }],
      };
    case 'toggled':
      return {
        ...state,
        todos: state.todos.map(t =>
          t.id === action.id ? { ...t, done: !t.done } : t
        ),
      };
    case 'deleted':
      return {
        ...state,
        todos: state.todos.filter(t => t.id !== action.id),
      };
    case 'filterChanged':
      return {
        ...state,
        filter: action.filter,
      };
    default:
      throw new Error(`Unknown action: ${action.type}`);
  }
}

For comprehensive useReducer patterns, see: research/react-19-comprehensive.md lines 506-521.

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