JSFiddle - React, Tailwind, and code Playground

by Peter Hozák

HTML

<input type="number" id="ctnQty" value="14" data-default-value="14">
<input type="number" id="ctnQty2" value="30" data-default-value="30">

JavaScript

// use jQuery or feature detection for cross-browser and future compatibility of handling the change event

var changeHandler = function(e) {
    var defaultValue = e.target.dataset.defaultValue;
    var v = e.target.value;
    if (v > defaultValue) {
        v = (Math.floor(v / defaultValue) + 1) * defaultValue;
        e.target.value = v;
    };
};

document.getElementById("ctnQty").onchange = changeHandler;
document.getElementById("ctnQty2").onchange = changeHandler;