JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="test" onkeypress="return isNumber(event)" />
<p id='demo'></p>

JavaScript

function isNumber(evt) {
    evt = (evt) ? evt : window.event;
    var charCode = (evt.which) ? evt.which : evt.keyCode;
    if (charCode > 31 && (charCode < 48 || charCode > 57)) {
        return false;
    }
    let inputValue = document.getElementById('test').value;
		let inputLength = inputValue.length;
		document.getElementById("demo").innerHTML = inputLength;
    return true;
}