JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" action="sendmail.php" onsubmit="return check_form()" >
	<input type="text" class="text required" value="" placeholder="nome" name="nome">
	<input type="text" class="text required" value="" placeholder="email" name="email">
	<input type="text" class="text required" value="" placeholder="telefone" name="telefone">
	<textarea class="required" placeholder="mensagem" name="mensagem"></textarea>
	<input type="submit" value="[ Enviar Mensagem ]" />
</form>

JavaScript

function check_form(){
	var inputs = document.getElementsByClassName('required');
  var len = inputs.length;
  var valid = true;
  for(var i=0; i < len; i++){
     if (!inputs[i].value){ valid = false; }
  }
  if (!valid){
  	alert('Por favor preencha todos os campos.');
    return false;
  } else { return true; }
}