Javascript question about objects

Hi all,

1) About the original question—which doesn't regard prototype inheritance in itself: "How can I have a built-in length property for an object"—I think the most obvious approach is to directly use an Array as the base object. Since any Array is an Object you can append methods and properties to it while keeping benefit from the whole array's API:

var myData = [];
myData.myProp = 'foo';
myData.myMeth = function(){ return [this.myProp, this.length, this.join('|')]; };
// Usage
// ---
myData[0] = 10;
myData[1] = 20;
alert( myData.myMeth() ); // => foo,2,10|20
// etc.

2) Now, if for whatever reason you cannot use the previous technique or need to deal with a precustomized Object structure such as:

var myObj = {
    items: [],
    meth: function(){ alert("Hello World!"); },
    // etc.
    };

then the challenge is more difficult for the reasons that Jeff mentioned above. You can anyway mimic a kind of 'binding' mechanism this way:

// The original obj
// ---
var myObj = {
    items: [],
    meth: function(){ alert("Hello World!"); }
    };
// Adding length as a 'fake property' (getter and setter)
// ---
(myObj.length = function F(n)
{
    if( !F.root )
        {
        (F.root=this).watch('length', function(_,ov,nv){ F(nv); return ov; });
        F.valueOf = function(){ return F().valueOf(); };
        F.toString = function(){ return F().toString(); };
        }
    if( 'undefined' == typeof n )
        n = F.root.items.length >>> 0;
    else
        F.root.items.length = n;
    return n;
}).call(myObj);

// Usage
// ---
myObj.meth(); // => Hello World!
myObj.items[0] = 10;
myObj.items[1] = 20;
alert( myObj.length ); // => 2
var x = myObj.length-1; // works too, thanks to F.valueOf
alert( x ); // => 1
myObj.length = 5; // testing the setter
alert( myObj.length ); // => 5
alert( myObj.items ); // => 10,20,,,

Of course it's totally unreasonable to implement such an artillery just for the convenience of writing myObj.length instead of myObj.length()—and myObj.length=x instead of myObj.length(x)—so I only suggest this for the sake of experimentation.

@+

Marc

Until ExtendScript supports ECMAScript 5—does anyone know if this is going to happen, by the way?—which includes getter and setter properties, calling a function is the only way to do it.

(Defining b() on the prototype, so that all the objects made from Test() will share the same function, is  the more efficient and accepted way to do it.)

function Test(){
    this.a = [];
}
Test.prototype.b = function () {
    return this.a.length;    
};
var x = new Test();
x.a[0] = 3;
x.a[1] = 3;
x.b();

Jeff