Untitled

$(document).ready(function(){
  var clases = [];
  var parent = $("body");
  var divs = parent.children();

  for(var i = 0; i < divs.length; i++){

    clases.push(divs[i].className)
  }

  console.log(clases);

});

<body>
    <div class="Clase_1 Clase_12"></div>
    <div class="Clase_2"></div>
    <div class="Clase_3"></div>
</body>

(3) ["Clase_1 Clase_12", "Clase_2", "Clase_3"]