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React useReducer Hook


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 two arguments.

useReducer(<reducer>, <initialState>)

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

The useReducer Hook returns the current stateand a dispatchmethod.

Here is an example of useReducer in a counter app:

Example:

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

const initialTodos = [
  {
    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 Todos() {
  const [todos, dispatch] = useReducer(reducer, initialTodos);

  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>
      ))}
    </>
  );
}

ReactDOM.render(<Todos />, document.getElementById('root'));

Run Example »


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.