drinks-manager/app/static/js/custom_number_input.js

{

    document.addEventListener("DOMContentLoaded", () => {
        // get all customnumberinput Elements
        let customNumberInputElements = document.getElementsByClassName("customnumberinput");
        // Add Event Handler to the elements of the customnumberinputs
        [...customNumberInputElements].forEach(element => {
            // number input
            let numberFieldElement = element.getElementsByClassName("customnumberinput-field")[0];
            // minus button
            element.getElementsByClassName("customnumberinput-minus")[0].addEventListener("click", () => {
                alterCustomNumberField(numberFieldElement, -1)
            });
            // plus button
            element.getElementsByClassName("customnumberinput-plus")[0].addEventListener("click", () => {
                alterCustomNumberField(numberFieldElement, +1)
            });
        })
    })

    function alterCustomNumberField(numberFieldElement, n) {
        numberFieldElement.value = Math.min(
            Math.max(
                (parseInt(numberFieldElement.value) + n), numberFieldElement.min || Number.MIN_VALUE
            ),
            numberFieldElement.max || Number.MAX_VALUE
        );
    }

}