What is normalized Redux state?

Normalized state stores data the way a relational database would: each type of entity is kept in a lookup table keyed by ID, instead of being nested/duplicated inside arrays. Redux Toolkit's createEntityAdapter generates this shape for you.

     // Un-normalized — posts are duplicated wherever they appear
     {
       posts: [
         { id: 1, title: "Post 1", author: { id: 1, name: "Sudheer" } },
         { id: 2, title: "Post 2", author: { id: 1, name: "Sudheer" } },
       ];
     }

     // Normalized — each entity stored once, referenced by id
     {
       posts: {
         ids: [1, 2],
         entities: {
           1: { id: 1, title: "Post 1", authorId: 1 },
           2: { id: 2, title: "Post 2", authorId: 1 },
         },
       },
       users: {
         ids: [1],
         entities: { 1: { id: 1, name: "Sudheer" } },
       },
     }
     

     import { createEntityAdapter, createSlice } from "@reduxjs/toolkit";

     const postsAdapter = createEntityAdapter();

     const postsSlice = createSlice({
       name: "posts",
       initialState: postsAdapter.getInitialState(),
       reducers: {
         postAdded: postsAdapter.addOne,
         postUpdated: postsAdapter.updateOne,
       },
     });