JSFiddle - React, Tailwind, and code Playground

HTML

<input type="textfield" id="input"/>
<input type="textfield" id="echo"/>

JavaScript

function countSymbols(string) {
    var regexAstralSymbols = /[\uD800-\uDBFF][\uDC00-\uDFFF]/g;
    return string
        // Replace every surrogate pair with a BMP symbol.
        .replace(regexAstralSymbols, '_')
        // …and *then* get the length.
        .length;
}

document.getElementById("input").onkeyup=function () {
	document.getElementById("echo").value=countSymbols(this.value);
}