Javascript 2D Array

Array2D.js

/* Extremely simple definition of a 2D array in Javascript. Javascript doesn't allow for a match-all getter so a length has to be defined. This defines the height or rows of the array but not the width; the width or columns is as big as you want it to be, sugar. For some odd reason, the __defineGetter__ doesn't send the name of the current "get" to the callback (which is silly in my opinion for situations exactly like these) so I had to convert the functions to strings. */

var Array2D = function(l) {
        if(!l) throw new Error("Please specify a length for your Array2D");
        this.arr = [];
        this.definedLength = l;

        for(var i = 0; i < l; i++) {
                this.__defineGetter__(i, Function("var i = " + i + ";if(!this.arr[i]) this.arr[i] = [];return this.arr[i];"));
        }

        this.__defineGetter__("length", function() {
                return this.arr.length;
        });

        this.__defineGetter__("definedLength", function() {
                return l;
        });

        return this;
};

Array2D.prototype.forEach = function(fn) {
        var that = this;
        this.arr.forEach(function(arr, y) {
                arr.forEach(function(val, x) {
                        fn.call(that, val, x, y);
                });
        });
};

/** Example Usage **/
var arr = new Array2D(40);

arr[20][10] = 5;
arr[5][3] = 6;

arr.length; //Returns 2, counts the amount of rows set
arr.definedLength; //Returns 40

arr.forEach(function(value, column, row) {
   //Yay
});