function property_exists (cls, prop) {
  // http://kevin.vanzonneveld.net
  // +   original by: Brett Zamir (http://brett-zamir.me)
  // *     example 1: function class_a () {this.prop1 = 'one'};
  // *     example 1: var instance_a = new class_a();
  // *     example 1: property_exists(instance_a, 'prop1');
  // *     returns 1: true
  // *     example 2: function class_a () {this.prop1 = 'one'};
  // *     example 2: var instance_a = new class_a();
  // *     example 2: property_exists(instance_a, 'prop2');
  // *     returns 2: false
  cls = (typeof cls === 'string') ? this.window[cls] : cls;

  if (typeof cls === 'function' && cls.toSource && cls.toSource().match(new RegExp('this\\.' + prop + '\\s'))) {
    // Hackish and non-standard but can probably detect if setting
    // the property (we don't want to test by instantiating as that
    // may have side-effects)
    return true;
  }

  return (cls[prop] !== undefined && typeof cls[prop] !== 'function') || (cls.prototype !== undefined && cls.prototype[prop] !== undefined && typeof cls.prototype[prop] !== 'function') || (cls.constructor && cls.constructor[prop] !== undefined && typeof cls.constructor[prop] !== 'function');
}

function class_a () {this.prop1 = 'one'};
var instance_a = new class_a();
property_exists(instance_a, 'prop1');

true

function class_a () {this.prop1 = 'one'};
var instance_a = new class_a();
property_exists(instance_a, 'prop2');

false