Redux.md

Redux

Reference: redux docs, velopert

1. What is Redux?

• The most widely used state management library in the React ecosystem
• By using Redux
  • You can separate state-related logic of components into different files for more efficient management
  • You can also easily manage global state

2. Redux vs Context API

The difference between using Redux and Context API

2-1. Middleware

• Redux has the concept of middleware
  • Using Redux middleware, you can perform desired tasks before an action object is processed by the reducer
    • ex)
      • Making an action be ignored based on specific conditions
      • Logging an action to the console
      • Modifying an action when it is dispatched so it is passed to the reducer in a different form
      • When a specific action occurs
        • Triggering other actions based on it
        • Executing specific JavaScript functions
• Middleware is primarily used for handling asynchronous operations

2-2. Useful Functions & Hooks

• Using the connect function, you can receive Redux state or action creator functions as component props
• Using Hooks like useSelector, useDispatch, useStore, you can easily query state or dispatch actions

2-3. Global State

• When managing global state using Context API, you typically create a separate Context for each feature,
  • With Redux, it is mandatory to put all global state into one large object
    • This helps avoid the inconvenience of creating a new Context every time!

3. Key Concepts

3-1. Action

• When a change in state is needed, an action is triggered

• An action is represented as a single object

  • ex)

    {
      type: "VALUE"
    }

• An action object must have a type as a required field, and other values can be added as desired

  • ex)

    {
      type: "TODO",
      data: {
        id: 0,
        text: "Go home"
      }
    }

  • ex)

    {
      type: "CHANGE_INPUT",
      text: "Please go home"
    }

3-2. Action Creator

• An action creator is literally a function that creates actions

• It takes parameters and creates them in the form of an action object

• ex)

  export function addTodo(data) {
    return {
      type: "ADD_TODO",
      data
    };
  }
  
  // Arrow function is also possible!
  export const changeInput = text => ({
    type: "CHANGE_INPUT",
    text
  });

• The reason for using Action Creators is to make it easier to trigger actions from components later

  • That's why the export keyword is typically attached to use the function from other files

• Using Action Creators is not mandatory in Redux!

  • You can also write action objects directly each time you trigger an action

3-3. Reducer

• A Reducer is a function that produces changes

• It takes two parameters

  • state

  • action

    • ex)

      function reducer(state, action) {
        // Update status
        return alteredState;
      }

• A Reducer takes the current state and the received action, creates and returns a new state based on them

3-4. Store

• In Redux, you create one store per application
• The store contains the current app state and the reducer, along with several built-in functions

3-5. Dispatch

• dispatch is one of the built-in functions of the store

• dispatch serves the role of triggering an action

  • An action is passed as a parameter to the dispatch function

    • ex)

      dispatch (action)

• When dispatch is called, the store executes the reducer, and if there is logic to handle that action, it creates a new state based on that action

3-6. Subscribe

• subscribe is also one of the built-in functions of the store, like dispatch
• If you pass a specific function to the subscribe function, that passed function is called every time an action is dispatched
• You can subscribe to the redux store's state using the connect function or useSelector Hook provided by the react-redux library

4. Three Principles

You must follow these 3 fundamental principles when using Redux

4-1. Single source of truth

• One application should create and use only one store
  • It is possible to use multiple stores, but it is not recommended!

4-2. State is read-only

• In React, when you need to update state, you use setState, and when you need to update an array, instead of pushing directly to the array, you use functions like concat to create a new array and replace it without modifying the existing array
• Similarly in Redux, by creating a new state to update rather than changing the existing state,
  • Later, you can go back or forward using developer tools
• The reason immutability must be maintained in Redux is that internally it performs shallow equality checks to detect data changes
  • This allows detecting changes in objects by performing a surface-level comparison rather than comparing deep inside the objects, enabling good performance

4-3. Changes are made with pure functions

• You should create pure functions while keeping the following 3 points in mind
  • Reducer functions take the previous state and action object as parameters
  • The previous state should never be touched; instead, create a new state object with the changes and return it
  • A reducer function called with the same parameters must always return the same result