How to bypass this magic?

Question

JavaScript

Wrote this code in vuex mutation:

ADD_TO_CART: function(state, payload) {
    let found = state.cartItems.find(i => i.id == payload.id);
    if (found){
        found.count = count + payload.count;
    }
     else {
        state.cartItems.push(payload);
    }
    window.localStorage.setItem('cart', JSON.stringify(state.cartItems));
  }

I get this magic: when there is already a product in the basket, but you need to add its quantity, then found.count = payload.count. That is, if before calling this function, found had count = 1, then call this function with payload.count = 5, then as soon as the found.count = 5 function begins, and after executing the line found.count += payload.count we will get found.count = 10. How does this even work?
Example (changed the code a little, but the result is the same):

//state.cartItems[index].count = 1, payload.count = 5 (под index подразумевается индекс объекта, который будем менять)
ADD_TO_CART: (state, payload) =>{
    //уже тут state.cartItems[index].count = 5, payload.count = 5 (а вот почему - хз)
    let index = state.cartItems.findIndex(i => i.id == payload.id);
    if (index > -1){
        state.cartItems[index].count += payload.count;
         //state.cartItems[index].count = 10, payload.count = 5
    }
     else {
        state.cartItems.push(payload);
    }
    window.localStorage.setItem('cart', JSON.stringify(state.cartItems));
  }

Answer 1

I don't quite understand why you don't get the error count is not defined

apples = 5;
basket = 0;

basket = apples; // тупо присваем в корзину яблоки
basket += apples; // прибавляем яблок к тем, что уже есть корзине
basket = basket + apples; // тоже самое что и выше

Yes, I don't quite understand the logic. You find a product in, roughly speaking, a general state and change its account, naturally, the next time you have this account will be increased, because you changed it in the state.