JSFiddle - React, Tailwind, and code Playground

HTML

<div id="output"></div>

<input id="testInput" type="text"></input>

CSS

#output{
   background: #fee;
   width: 200px;
   height: 25px;
   margin: 15px 0;
   color: #a33;
}

JavaScript

function isNumber(n){
  return (parseFloat(n) == n);
}


function print(msg){
    $("#output").html(msg + "</br>");
}

$("document").ready(function(){
    $("#testInput").keyup(function(event){
        var input = $(this).val();
        if(!isNumber(input)){
            print("enter numbers only");
            $(this).val(input.substring(0, input .length-1));
        }else
            print("");
    });
});