JAVASCRIPT

usereducer hook react

import React, { useReducer } from 'react'
import { render } from 'react-dom'

const types = {
  PET: 'PET',
  COLOR: 'COLOR',
}

const reducer = (state, action) => {
  switch (action.type) {
    case types.PET:
      return { ...state, pet: action.value }
    case types.COLOR:
      return { ...state, color: action.value }
  }
}

const initialState = {
  color: 'black',
  pet: 'cat',
}

export default function App() {
  const [state, dispatch] = useReducer(reducer, initialState)

  return (
    <div>
      <label>Choose a color and a pet: </label>
      <br />
      <select
        value={state.color}
        onChange={event => {
          dispatch({ type: types.COLOR, value: event.target.value })
        }}
      >
        <option value="black">Black</option>
        <option value="pink">Pink</option>
        <option value="blue">Blue</option>
      </select>
      <select
        value={state.pet}
        onChange={event => {
          dispatch({ type: types.PET, value: event.target.value })
        }}
      >
        <option value="cat">Cat</option>
        <option value="dog">Dog</option>
        <option value="mouse">Mouse</option>
      </select>
      <br />
      <br />
      You chose a {state.color} {state.pet}
    </div>
  )
}

render(<App />, document.querySelector('#app'))

useReducer Hooks

const [state, dispatch] = useReducer(reducer, initialArg, init);

const initialState = {count: 0};

function reducer(state, action) {
  
  switch (action.type) {
    case 'increment':
      return {count: state.count + 1};
    case 'decrement':
      return {count: state.count - 1};
    default:
      throw new Error();
  }
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);
  return (
    <>
      Count: {state.count}
      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
      <button onClick={() => dispatch({type: 'increment'})}>+</button>
    </>
  );
}
const [state, dispatch] = useReducer(
    reducer,
    {count: initialCount}
  );

React useReducer Hook

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

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>
      ))}
    </>
  );
}

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

useReducer() hook react

code snippet 

https://codesandbox.io/s/usereducer-demo-zxguu?file=/src/App.js

usestate or usereducer

//When it's just an independent element of state you're managing

useState