code: function Overlay(opt) {
      var opt = opt || {};
      opt.zIndex = opt.zIndex || '200';
      opt.trans = opt.trans || '0.6';
      if(document.getElementsByClassName('cl-overlay').length > 0) return;
      var ol = this.el = document.body.appendChild(createOverlay(opt.zIndex, opt.trans));
  }
docs: Constrcutor
  @param {Number} zIndex Description
  @return {Element} description