Arve Bersvendsen

The mystery argument

Consider this:

window.setTimeout(foo,100,"bar","baz");

function foo(){
  alert(arguments.length);
  for (var arg = 0;arguments[arg];arg++){
    alert(arg+": "+arguments[arg]);
  }
}

What should the value of arguments.length be?

The documentation I use every day, suggest that you can pass any number of arguments in a window.setTimeout and have them available from the arguments array. However, that is not the case:

Internet Explorer

Internet Explorer won’t let me pass arguments to a setTimeout, and makes arguments.length == 0; — which is fair enough, The MSDN documentation suggests that only the optional lang parameter is allowed

Opera

Opera tells me that arguments.length == 2 which is exactly what I expected.

Mozilla/Firefox

And along comes the mystery: The Gecko DOM Reference states that no optional arguments are allowed. Supporting “language” doesn’t make much sense when you have only one scripting language.

However, in Gecko, arguments.length == 3 — where the first two values are arguments[0] == "bar"; and arguments[1] == "baz";. The last one, however is a mystery: It returns a numeric value. The value of arguments[2] is 10, 13 and 7, respectively.

Can anyone please enlighten me and tell me what this third value of the arguments array is, why it’s there and where it’s documented?