<link href="//maxcdn.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css" rel="stylesheet" id="bootstrap-css">
<script src="//maxcdn.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<!------ Include the above in your HEAD tag ---------->
<div class="container">
<div class="row">
<h2>Menu JS</h2>
</div>
</div>
<form cadastro action="">
<label for = "venda"> Selecione uma opção</label>
<Select id="venda">
<option value="0">Escolha uma Opção</option>
<option value="1">Cadastrar Comprador</option>
<option value="2">Cadastrar Automóvel</option>
<option value="3">Cadastrar venda</option>
<option value="4">Sair do sistema</option>
<script>
var select = document.querySelector('select');
var para = document.querySelector('p');
select.addEventListener('change',setVenda);
function setVenda(){
var choice = select.value;
if (choice === "1"){
para.textContent = 'Menu de Cadastro do comprador';
calcula();
}else if_(choise ==="2"){
para.textContent = 'Menu de Cadastro do Automóvel';
}else if_(choise ==="3"){
para.textContent = 'Menu de Cadastro do Vendas';
}else if_(choise ==="4"){
para.textContent = 'Sair';
}
else{
para.textContent = '';
}
}
function calcula(){
alert("eu sou um alerta !");
}
</script>
</form>
