How to retrieve specific pieces of state using selectors

In Redux, there are some techniques used to retrieve specific pieces of state from the Redux store.

In this article, we will create a to-do list application using Redux and React, and we use the Reselect library for optimized state selection. This app provides basic functionalities such as adding new to-dos and displaying a list of existing to-dos. This serves a practical knowledge to retrieve a specific piece of state from the Redux store.

Approach to retrieve state

Project Setup: The project follows a file structure for a React application created with create-react-app. This includes separate files and folders for components, Redux store, and React configuration.
Create App: Create a new React project using Create React App and install Redux and React Redux to manage the application state.
State Management: Code of Reducers, actions, and selectors are organized in one file.
Component: Create a functional component ( App.js ) main component and connect it to the Redux store.
Styling: Create a CSS file ( App.css ) for styling the input field, buttons, and to-do list.

Steps to Create Application

Step 1: Create React Application named redux-selector-app and navigate to it using this command.

npx create-react-app redux-selector-app
cd redux-selector-app

Step 2: Install required packages and dependencies.

npm install  react react-redux redux reselect

Updated dependencies

Updated dependencies will look like in package.json file.

"dependencies": {
    "react": "^18.2.0",
    "react-dom": "^18.2.0",
    "react-redux": "^9.1.1",
    "redux": "^5.0.1",
    "reselect": "^5.1.0",
  },

Project Structure:
Project Structure

Example: The following codes implements the above approach.

CSS

/* App.css */

#app {
    text-align: center;
    display: flex;
    margin-top: 20px;
    justify-content: center;
}

button {
    padding: 10px 20px;
    background-color: rgba(220, 224, 219, 0.87);
    border: 1px solid black;
    cursor: pointer;
}

button:hover {
    background-color: rgba(4, 163, 4, 0.637);
}

input {
    padding: 10px 20px;
}

li {
    list-style: none;
}

JavaScript

// store.js

import { createStore, combineReducers } from 'redux';
import { createSelector } from 'reselect';

// Reducers
const todosReducer = (state = { todos: [] }, action) => {
    switch (action.type) {
        case 'ADD_TODO':
            return {
                ...state,
                todos: [...state.todos, action.payload],
            };
        default:
            return state;
    }
};

// Actions
export const addTodo = todo => ({
    type: 'ADD_TODO',
    payload: todo,
});

// Selectors
const getTodosState = state => state.todos;

export const getTodos = createSelector(
    [getTodosState],
    todosState => todosState.todos
);

// Combine Reducers
const rootReducer = combineReducers({
    todos: todosReducer,
});

// Create Redux Store
const store = createStore(rootReducer);

export default store;

JavaScript

// index.js 

import React from "react";
import ReactDOM from "react-dom/client";
import App from "./App";
import { Provider } from "react-redux";
import store from "./store";

const root = ReactDOM.createRoot(document.getElementById("root"));
root.render(
    <Provider store={ store } >
    <App />
< /Provider>
);

JavaScript

// App.js

import React, { useState } from "react";
import { connect } from "react-redux";
import { addTodo, getTodos } from "./store";
import "./App.css";

const App = ({ todos, addTodo }) => {
  const [newTodo, setNewTodo] = useState("");

  const handleAddTodo = () => {
    if (newTodo.trim() !== "") {
      addTodo({
        id: Date.now(),
        text: newTodo,
        completed: false,
      });
      setNewTodo("");
    }
  };

  return (
    <div id="app">
      <div>
        <h1>
          <img
            src="https://media.geeksforgeeks.org/gfg-gg-logo.svg"
            alt="gfg_logo"
          />
          Todo List{" "}
        </h1>

        <input
          type="text"
          value={newTodo}
          onChange={(e) => setNewTodo(e.target.value)}
          placeholder="Enter a new todo..."
        />
        <button onClick={handleAddTodo}>Add Todo</button>
        <ul>
          {todos.map((todo) => (
            <li key={todo.id}>{todo.text}</li>
          ))}
        </ul>
      </div>
    </div>
  );
};

const mapStateToProps = (state) => ({
  todos: getTodos(state),
});

const mapDispatchToProps = {
  addTodo,
};

export default connect(mapStateToProps, mapDispatchToProps)(App);