Can someone please explain me this code from ReactJS? - reactjs

Question

can anyone please explain me this code? I am not able to understand as in what's happening here.
const cartReducer = (state, action) => {
if (action.type === "ADD") {
const updatedTotalAmount =
state.totalAmount + action.item.price * action.item.amount;
const existingCartItemIndex = state.items.findIndex(
(item) => item.id === action.item.id
);
const existingCartItem = state.items[existingCartItemIndex];
let updatedItems;
if (existingCartItem) {
const updatedItem = {
...existingCartItem,
amount: existingCartItem.amount + action.item.amount,
};
updatedItems = [...state.items];
updatedItems[existingCartItemIndex] = updatedItem;
} else {
updatedItems = state.items.concat(action.item);
}
return {
items: updatedItems,
totalAmount: updatedTotalAmount,
};
}
return defaultCartState;
};

Answer 1

That is a redux reducer. Please read this tutorial to get familiar with the concepts of it:
https://redux.js.org/tutorials/fundamentals/part-3-state-actions-reducers

Answer 2

Reducers were popularized by Redux but are not a concept inherent to Redux in the sense that you can write a reducer without any import from Redux. A reducer is a concept for a particular kind of function i.e.:
a function that receives the current state and an action object, decides how to update the state if necessary, and returns the new state: (state, action) => newState. "Reducer" functions get their name because they're similar to the kind of callback function you pass to the Array.reduce() method.
Source: Redux docs
React now comes with a useReducer hook built-in. See Hooks API Reference.

Answer 3

I have added some comments to your code I hope this makes the code a bit more understandable.
const cartReducer = (state, action) => {
// Adding an Item to Cart
if (action.type === "ADD") {
// Calculated Cart Total: existing Total + (new Item Price * new item Quantity)
const updatedTotalAmount = state.totalAmount + action.item.price * action.item.amount;
/*
* Finding Items Index in the Cart Array using the Item ID.
* Index will be Returned only if Item with same od already exist otherwise -1
*/
const existingCartItemIndex = state.items.findIndex((item) => item.id === action.item.id);
/*
* Getting the CartItem Based on the Index.
* If the value is -1 i.e., item already doesn't exist, then this code will return undefined
*/
const existingCartItem = state.items[existingCartItemIndex];
let updatedItems;
// existingCartItem will have an Object(which evaluates to true) only if Item already existed in Cart
if (existingCartItem) {
// Creating updatedItem by spreading the existingItems data + updating amount/Quantity to: existing Quantity + new Quantity
const updatedItem = {
...existingCartItem,
amount: existingCartItem.amount + action.item.amount,
};
// Making a Copy of Items Array & Replacing Existing Item with new/updated entry
updatedItems = [...state.items];
updatedItems[existingCartItemIndex] = updatedItem;
} else {
// If the Item doesn't already exist in Cart then we Just add that New Item to the Cart
updatedItems = state.items.concat(action.item);
}
// Return the State with Updated Items List & total Amount
return {
items: updatedItems,
totalAmount: updatedTotalAmount,
};
}
// Default State Return
return defaultCartState;
};