function func_get_arg (num) {
  // http://kevin.vanzonneveld.net
  // +   original by: Brett Zamir (http://brett-zamir.me)
  // %        note 1: May not work in all JS implementations
  // *     example 1: function tmp_a() {return func_get_arg(1);}
  // *     example 1: tmp_a('a', 'b');
  // *     returns 1: 'a'
  if (!arguments.callee.caller) {
    try {
      throw new Error('Either you are using this in a browser which does not support the "caller" property or you are calling this from a global context');
      //return false;
    } catch (e) {
      return false;
    }
  }

  if (num > arguments.callee.caller.arguments.length - 1) {
    try {
      throw new Error('Argument number is greater than the number of arguments actually passed');
      //return false;
    } catch (e2) {
      return false;
    }
  }

  return arguments.callee.caller.arguments[num];
}

function tmp_a() {return func_get_arg(1);}
tmp_a('a', 'b');

'a'