function call_user_func (cb) {
  // http://kevin.vanzonneveld.net
  // +   original by: Brett Zamir (http://brett-zamir.me)
  // +   improved by: Diplom@t (http://difane.com/)
  // +   improved by: Brett Zamir (http://brett-zamir.me)
  // *     example 1: call_user_func('isNaN', 'a');
  // *     returns 1: true
  var func;

  if (typeof cb === 'string') {
    func = (typeof this[cb] === 'function') ? this[cb] : func = (new Function(null, 'return ' + cb))();
  } 
  else if (Object.prototype.toString.call(cb) === '[object Array]') {
    func = (typeof cb[0] === 'string') ? eval(cb[0] + "['" + cb[1] + "']") : func = cb[0][cb[1]];
  }
  else if (typeof cb === 'function') {
    func = cb;
  }

  if (typeof func !== 'function') {
    throw new Error(func + ' is not a valid function');
  }

  var parameters = Array.prototype.slice.call(arguments, 1);
  return (typeof cb[0] === 'string') ? func.apply(eval(cb[0]), parameters) : (typeof cb[0] !== 'object') ? func.apply(null, parameters) : func.apply(cb[0], parameters);
}

call_user_func('isNaN', 'a');

true