function class_exists (cls) {
  // http://kevin.vanzonneveld.net
  // +   original by: Brett Zamir (http://brett-zamir.me)
  // *     example 1: function class_a() {this.meth1 = function () {return true;}};
  // *     example 1: var instance_a = new class_a();
  // *     example 1: class_exists('class_a');
  // *     returns 1: true
  var i = '';
  cls = this.window[cls]; // Note: will prevent inner classes
  if (typeof cls !== 'function') {
    return false;
  }

  for (i in cls.prototype) {
    return true;
  }
  for (i in cls) { // If static members exist, then consider a "class"
    if (i !== 'prototype') {
      return true;
    }
  }
  if (cls.toSource && cls.toSource().match(/this\./)) {
    // Hackish and non-standard but can probably detect if setting
    // a property (we don't want to test by instantiating as that
    // may have side-effects)
    return true;
  }

  return false;
}

function class_a() {this.meth1 = function () {return true;}};
var instance_a = new class_a();
class_exists('class_a');

true