<div class="shopcart__item-quanity">
<select class="cart_qty">
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
<option>5</option>
</select>
</div>
<div class="shopcart__item-unitprice">
1340
</div>
<div class="shopcart__item-discount">
200
</div>
<div class="shopcart__item-total" data-price = "1140">
1140
</div>
$('.cart_qty').change(function() {
var qty = this.value;
console.log(qty)
var total = $('.shopcart__item-total').attr('data-price');
sum = Number(total) * qty;
console.log(sum);
$('.shopcart__item-total').html(sum);
});
const wrapperSelector = 'селектор блока с товаром, вам виднее, каким он должен быть';
const quantitySelector = '.cart_qty';
const totalSelector = '.shopcart__item-total';
$(wrapperSelector).on('change', quantitySelector, e => {
$(totalSelector, e.delegateTarget).text(function() {
return e.target.value * this.dataset.price;
});
});
// или
document.querySelectorAll(quantitySelector).forEach(function(n) {
n.addEventListener('change', this);
}, ({ target: t }) => {
const total = t.closest(wrapperSelector).querySelector(totalSelector);
total.innerText = t.value * total.dataset.price;
});
<div class="shopcart__item-quanity">
<select class="cart_qty">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>
</div>
<div class="shopcart__item-unitprice">
1340
</div>
<div class="shopcart__item-discount">
200
</div>
<div class="shopcart__item-total" data-price = "1140">
1140
</div>
$('.cart_qty option').change(function() {
var qty = this.value;
console.log(qty)
var total = $('.shopcart__item-total').attr('data-price');
sum = parseInt(total) * qty;
console.log(sum);
$('.shopcart__item-total').html(sum);
});