alterar cor a alterar campo

by Rapha Souza

HTML

<script src="//code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<link rel="stylesheet" href="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="//cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>

</style>
</head>
<body>
<br>
<label>Nmr Atividade</label>
<input type="text" class="form-control " id="atividadePosterior" >
<br>
<button onclick="myFunction()">Try it</button>
This is a DIV element.
<input type="text" class="form-control " id="Valor" onchange="campoAlterado()" >



<script>
function campoAlterado() {
   var nmratividade = document.getElementById("atividadePosterior").value;
   var element = document.getElementById("Valor");
 
   
   if (nmratividade == "20"){
  
      element.classList.add("alert-danger");
    //  alert("aviso");
   
   }
   
   
   
}
</script>

</body>
</html>