undefined [JavaScript snippet]

function main() {
  var undef;

  txt_out("undef: " + undef);      // undef: undefined

  txt_out(2+undef);                // NaN

  txt_out(undefined == undefined); // true
  txt_out(undefined == undef    ); // true
  txt_out(undefined == null     ); // true
  txt_out(undefined == 2        ); // false
}