const amount = document.getElementById("amount");
function setAmount(btn) {
    let id = btn.getAttribute("id");
    let min = amount.getAttribute("min");
    let max = amount.getAttribute("max");
    let step = amount.getAttribute("step");
    let value = amount.getAttribute("value");
    let calcStep = (id == "button_increment") ? (step * 1) : (step * -1);
    let newAmount = parseInt(value) + calcStep;

    if (newAmount >= min && newAmount <= max) {
        amount.setAttribute("value", newAmount);
    }
}