JSFiddle - React, Tailwind, and code Playground

HTML

Al perder foco el textarea te dirá la cantidad de caracteres
<textarea id="editor"></textarea>

JavaScript

function contarCaracteresDelTexto () {
    var txt = document.getElementById('editor');
    var div = document.createElement('div');
    div.innerHTML = txt.value;
    if(div.textContent) {
        return div.textContent.length;
    } else {
       return div.innerText.length;
    }
}

$('#editor').blur(function(){ alert(contarCaracteresDelTexto())});