JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>

<form id="contact-form">
<input type="text" name="nome" placeholder="Nome*:" value="" class="required" />
<input type="text" name="telefone" placeholder="Telefone*:" value="" class="required" />
<input type="text" name="email" placeholder="Email*:" value="" class="required" />
<button type="submit" id="enviar"  name="submit">Concluir Cadastro</button> 
</form>

JavaScript

$("#contact-form").submit(function(){

    event.preventDefault();

    var dados = $(this).serialize();
    var campos = $(this).find('.required');

    $(campos).each(function() {
        for(i=0; i=$(this).val() == ''; i++ ){
            if( $(this).val() == '' )
            {
                alert("Preencha os campos obrigatórios");
                $(this).focus();
                e.preventDefault();
            }else {
                 $.ajax({
                     type: "POST",
                     url: "cadastrar.php",
                     data: dados,
                     success: function(data)
                     {
                         $("#status").slideDown();
                         $("#status").html(data);
                     }
                 });
            $('#contact-form').trigger("reset");

            }
        }
    });
});