Untitled

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"/>
<title>ot4d</title>
</head>
<body>
  <input id="postiNumero">
  <button onclick="x();">Tarkasta</button>
  <script>
  function x(){
    var posti_nro = document.getElementById("postiNumero").value;
    var check =  console.log(tarkastaPostiNumero(posti_nro));

  function tarkastaPostiNumero(numero) {
    if(posti_nro.toString().length == 5){
      return true;
    }
    else {
      return false;
    }
  }
}
  </script>
</body>