How to make a selection from an object?

Question

JavaScript

It is necessary to add one element to "localstorage", let's say "{2021:1}" , but not all at once.

var items = { 2021: 0, 2022: 0, 2023: 0, 2024: 0 };

out1 += '<button class="number" data-art="' + key + '">' + items[key] + '</button>';

function plus() {
    var articul = $(this).attr('data-art');
    if (cart[articul] != undefined) {
        cart[articul]++;
    }
    else {
        cart2[articul] = 1;
    }
    localStorage.setItem('cart', JSON.stringify(cart)); 
}

Answer 1

So what's the problem?

localStorage.setItem('cart', JSON.stringify({[2021]: cart[2021]}));