JSFiddle - React, Tailwind, and code Playground

HTML

<p>Encore <strong id="reception_num_carac">350</strong> caractères</p>
<textarea class="charcounter" maxlength="350" name='description' cols='55' rows='8' onkeyup="updateCharCounter(this, '#reception_num_carac')" ></textarea>

JavaScript

window.updateCharCounter = function(textarea, counterElementSelector){
  var numberOfLineBreaks = (textarea.value.match(/\n/g)||[]).length;
  var characterCount = textarea.value.length + numberOfLineBreaks;
  var restants = +(textarea.getAttribute("maxlength"))-characterCount;
  var counterElement = document.querySelector(counterElementSelector);
  if(counterElement != null){
     counterElement.style.color = (restants <= 10 ? "red" : "black");
     counterElement.textContent = restants;
  }
};