React useReducer

useReducer() is a method from the React Hooks API, similar to useState but gives you more control to manage the state. It takes a reducer function and initial state as arguments and returns the state and dispatch method: const [state, dispatch] = React.

• 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.

In React, useReducer essentially accepts a reducer function that returns a single value: const [count, dispatch] = useReducer(reducer, initialState); The reducer function itself accepts two parameters and returns one value. The first parameter is the current state, and the second is the action.

Syntax

The useReducer Hook accepts two arguments.

useReducer(<reducer>, <initialState>)

The reducer function contains your custom state logic and the initialState can be a simple value but generally will contain an object.

The useReducer Hook returns the current state and a dispatch method.

Here is an example of useReducer in a counter app:

import { useReducer } from "react";
import ReactDOM from "react-dom/client";

const initialTodo = [
  {
    id: 1,
    title: "Todo 1",
    complete: false,
  },
  {
    id: 2,
    title: "Todo 2",
    complete: false,
  },
];

const reducer = (state, action) => {
  switch (action.type) {
    case "COMPLETE":
      return state.map((todo) => {
        if (todo.id === action.id) {
          return { ...todo, complete: !todo.complete };
        } else {
          return todo;
        }
      });
    default:
      return state;
  }
};

function Todo() {
  const [todos, dispatch] = useReducer(reducer, initialTodo);

  const handleComplete = (todo) => {
    dispatch({ type: "COMPLETE", id: todo.id });
  };

  return (
    <>
      {todos.map((todo) => (
        <div key={todo.id}>
          <label>
            <input
              type="checkbox"
              checked={todo.complete}
              onChange={() => handleComplete(todo)}
            />
            {todo.title}
          </label>
        </div>
      ))}
    </>
  );
}

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<Todo />);

This is just the logic to keep track of the todo complete status.

All of the logic to add, delete, and complete a todo could be contained within a single useReducer Hook by adding more actions.