JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="cnfMobileNo" id="cnfMobileNo">

JavaScript

var el=document.getElementById('cnfMobileNo');
el.onkeydown=function onlyNumbers(evt) {
    var theEvent = evt || window.event,
        key = theEvent.keyCode || theEvent.which,
        exclusions=[8,9];/*Add exceptions here */
    if(exclusions.indexOf(key)>-1){return;}
    key = String.fromCharCode(key);
    var regex = /[0-9]|\./;
    if( !regex.test(key) ) {
        theEvent.returnValue = false;
        if(theEvent.preventDefault) theEvent.preventDefault();
    }
}