useReducer.md

id	title	sidebar_label	sidebar_position	tags
useReducer hook	useReducer Hook Concept	useReducer Hook	4	react create-react-app useReducer hooks react-scripts react-dom react-app

useReducer

Explanation: useReducer is an alternative to useState for managing complex state logic within a component. It is often preferable when you have state transitions that involve multiple sub-values or when the next state depends on the previous one.

When you call useReducer, you pass it a reducer function and an initial state. It returns an array with the current state (state) and a dispatch function (dispatch) to trigger state updates based on action objects.

Example:

import React, { useReducer } from 'react';

// Initial state with a 'count' property initialized to 0
const initialState = { count: 0 };

// Reducer function defines how state updates in response to actions
function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 }; // Increment 'count'
    case 'decrement':
      return { count: state.count - 1 }; // Decrement 'count'
    default:
      throw new Error(); // Throw an error for unknown actions
  }
}

function Counter() {
  // Use 'useReducer' with 'reducer' function and 'initialState'
  const [state, dispatch] = useReducer(reducer, initialState);

  return (
    <div>
      <p>Count: {state.count}</p>
      {/* Buttons dispatch 'increment' and 'decrement' actions */}
      <button onClick={() => dispatch({ type: 'increment' })}>Increment</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>Decrement</button>
    </div>
  );
}

Count: 0

{ document.getElementById("countValue").textContent=Number(document.getElementById("countValue").textContent)+1 }}>Increment   { document.getElementById("countValue").textContent=Number(document.getElementById("countValue").textContent)-1 }}>Decrement

In this example, useReducer manages state updates for count. dispatch is used to trigger actions ({ type: 'increment' } or { type: 'decrement' }), which are processed by the reducer function to compute the next state.