Question

It’s possible to set the individual elements of a function arguments property (what Mozilla calls an ‘array-like’ property), however Mozilla reports it is not possible to add elements to this property in SpiderMonkey 1.5 though this is fixed in 1.6 (ref. the note on SpiderMonkey here… https://developer.mozilla.org/en/JavaScript/Reference/Functions_and_function_scope/arguments).

This is a useful property, chaining constructors from subclasses, creating a list of arguments to pass to a function (e.g., myclassmethod.apply(this, arguments)), etc.

However I’ve found V8 will not extend the length in the same way Mozilla reports of SpiderMonkey 1.5. (Not sure what the status is with other JavaScript engines, Opera, Rhino, etc.).

Is this actually an ECMA feature? Has Mozilla got it wrong in considering this a bug, or does V8 have a bug to fix?

[Update] I’ve found using V8 that the arguments.length property can be assigned to, and so effectively arguments can be extended (or set to whatever length you require otherwise). However JSLint does complain this is a bad assignment.

[Update] Some test code if anyone wants to try this in Opera, FF etc., create an instance of a subclass calling the constructor with one arg while adding an element to arguments in the subclass constructor and calling the superclass constructor, the superclass should report two arguments:

function MyClass() {
    if (arguments.length) {
        console.log("arguments.length === " + arguments.length);
        console.log("arguments[0] === " + arguments[0]);
        console.log("arguments[1] === " + arguments[1]);
    }
}

function MySubClass() {
    console.log(arguments.length);
    //arguments.length = 2;  // uncomment to test extending `length' property works
    arguments[1] = 2;
    MyClass.apply(this, arguments);
}

MySubClass.prototype = new MyClass();

new MySubClass(1);

[Update] JSLint actually complains when you make any kind of assignment to arguments by the looks (e.g., arguments[0] = "foo"). So maybe JSLint has some work to do here also.

Answer 1

Not sure if this is what you are looking for, but you can make the arguments object into a standard array:

var args = [].slice.call(arguments, 0);

[EDIT] so you could do:

myclassmethod.apply(this, [].slice.call(arguments, 0).push(newValue));