recursively concatenating a javascript functions arguments

arguments is said to be an Array-like object. As you already saw you may access its elements by index, but you don't have all the Array methods at your disposal. Other examples of Array-like objects are HTML collections returned by getElementsByTagName() or getElementsByClassName(). jQuery, if you've ever used it, is also an Array-like object. After querying some DOM objects, inspect the resulting jQuery object with Firebug in the DOM tab and you'll see what I mean.

Here's my solution for the Meebo problem:

function conc(){
    if (arguments.length === 0)
        return "";
    else
        return Array.prototype.slice.call(arguments).join(" ");
}

alert(conc("a", "b", "c"));

Array.prototype.slice.call(arguments) is a nice trick to transform our arguments into a veritable Array object. In Firefox Array.slice.call(arguments) would suffice, but it won't work in IE6 (at least), so the former version is what is usually used. Also, this trick doesn't work for collection returned by DOM API methods in IE6 (at least); it will throw an Error. By the way, instead of call one could use apply.

A little explanation about Array-like objects. In JavaScript you may use pretty much anything to name the members of an object, and numbers are not an exception. So you may construct an object that looks like this, which is perfectly valid JavaScript:

var Foo = {
    bar : function() {
        alert('I am bar');
    },

    0 : function() {
        alert('I am 1');
    },

    length : 1
}

The above object is an Array-like object for two reasons:

1. It has members which names are numbers, so they're like Array indexes
2. It has a length property, without which you cannot transform the object into a veritable Array with the construct: Array.prototype.slice.call(Foo);

The arguments object of a Function object is pretty much like the Foo object, only that it has its special purpose.

Mozilla on the subject:

The arguments object is not an array. It is similar to an array, but does not have any array properties except length. For example, it does not have the pop method. However it can be converted to an real array:

var args = Array.prototype.slice.call(arguments);

Therefore the solution to your problem is fairly simple:

var string = Array.prototype.slice.call(arguments).join("");

BTW: It further states:

The arguments object is a local variable available within all functions; arguments as a property of Function can no longer be used.

You should only use arguments instead of func.arguments

This works:

function concatenate(){
    return [].join.call(arguments, "");
}
alert(concatenate("one", "two", "three"));