Broken code.

gistfile1.js

FMWK.button = function() {
  var _this = this;
  this.el = document.createElement('button');
  ['hide','show'].forEach(function(method){
    FMWK.button.prototype[method] = function(){
      $(_this.el)[method].apply($(_this.el), Array.prototype.slice.call(arguments));
     }
  });

  return this;
}

var b1 = new FMWK.button();
var b2 = new FMWK.button();

b1.hide() // fires on b2

The problem is _this.el is not closed over correctly, however wrapping it in a function and passing it in looses the arguments from the outer scope.

Every time you construct a new instance of button, you are overwriting the button prototype with new show and hide methods and hard coding _this to those methods. Set the show/hide methods outside of the constructor and reference this.el directly in there.