What is a Reducer?

• A reducer is a pure function that takes the current state of an application and an action object, and returns the new state.
• It’s responsible for handling state updates in a predictable and centralized way.
• It’s often used with the useReducer hook in React, or as a core concept in state management libraries like Redux.

How It Works:

1. Action Dispatched: When an event occurs (e.g., button click, data fetch), an action object is dispatched, describing the intended state change.
2. Reducer Called: The reducer receives the current state and the action object.
3. New State Calculated: The reducer, based on the action type and state, computes the new state immutably.
4. State Updated: The component’s state is updated with the new state returned by the reducer.
5. Component Re-renders: React re-renders the component with the updated state.

Example:

JavaScript

import React, { useReducer } from 'react';

function todoReducer(state, action) {
  switch (action.type) {
    case 'ADD_TODO':
      return [...state, action.payload];
    case 'TOGGLE_TODO':
      return state.map((todo) =>
        todo.id === action.payload.id ? { ...todo, completed: !todo.completed } : todo
      );
    default:
      return state;
  }
}

function TodoList() {
  const [todos, dispatch] = useReducer(todoReducer, []);

  // ... rest of the component
}

Pros:

• Centralized State Management: Reducers consolidate state logic, making it easier to manage and understand.
• Predictable Updates: State updates are deterministic, making debugging and reasoning about changes simpler.
• Testability: Reducers, as pure functions, are straightforward to test in isolation.
• Complex State Handling: Reducers excel at managing intricate state relationships and dependencies.
• Scalability: Reducers can be combined and composed to manage large applications effectively.

Cons:

• Boilerplate: For simple states, useReducer might introduce more code than useState.
• Learning Curve: Understanding reducers and their patterns requires some initial effort.
• Overkill for Simple Cases: Not always necessary for straightforward state management.

Best Practices:

• Keep reducers pure and avoid side effects.
• Use clear and concise action types.
• Structure reducers for readability and maintainability.
• Combine reducers for complex state using techniques like combineReducers.
• Consider using a state management library like Redux for large-scale applications.