function to append childrens on a parent
function appendChild(parent, childrens) {
  if (childrens.length === undefined) childrens = [childrens];
  var docFrag = document.createDocumentFragment(); //we used for loop instead of forEach because childrens can be array like object

  for (var i = 0, ln = childrens.length; i < ln; i++) {
    docFrag.appendChild(childrens[i]);
  }

  parent.appendChild(docFrag);
}