JSFiddle - React, Tailwind, and code Playground

HTML

<div id="test2" class="col-md-6 form-group">
  <form action="addcomment.php" method="post" name="guest" onSubmit="return fcheck()">

    <div class="textform">
      <p>Name:
        <input type="text" name="name" class="form-control" />
      </p>
    </div>

    <div class="textform">
      <p>E-Mail:
        <input type="text" name="email" class="form-control" />
      </p>
    </div>

    <div class="textform">
      <p>Nachricht(Maximal 300 Zeichen):</p>
      <textarea cols="50" name="message" rows="10" class="form-control status-box" maxlength="300"> </textarea>
    </div>
    <br>
    <div class="pull-left">
      <p class="counter">300</p>
    </div>

    <div id="textarea" class="pull-right">
      <input type="submit" value="Nachricht abschicken" class="btn btn-primary" />
    </div>
  </form>
</div>

JavaScript

function fcheck() {
  alert("Text area length : " + document.forms.guest.message.value.length)
  if (document.forms.guest.name.value.length < 1) {
    alert("Du musst einen Namen eingeben!");
    return false;
  }
  if (document.forms.guest.email.value.length < 1) {
    alert("Du musst eine Email eingeben!");
    return false;
  }
  if (document.forms.guest.message.value.length < 1) {
    alert("Du musst eine Nachricht eingeben!");
    return false;
  }
  return false;
}