ラムダ（あとで消す）

gistfile1.js

if (! Array.prototype.filter) {
    Array.prototype.filter = function (patternFunc) {
        var helper = function (thisArray, newArray) {
            if (! newArray) newArray = [];

            if (thisArray && thisArray.length > 0) {
                var el = thisArray.shift();
                if (patternFunc(el)) newArray.push(el);
                return helper(thisArray, newArray);
            }
            return newArray;
        };

        return helper(this.slice(0));
    };
}

if (! Array.prototype.map) {
    Array.prototype.map = function (patternFunc) {
        var helper = function (thisArray, newArray) {
            if (! newArray) newArray = [];

            if (thisArray && thisArray.length > 0) {
                var el = thisArray.shift();
                return helper(thisArray, newArray.push(patternFunc(el)));
            }
            return newArray;
        };

        return helper(this.slice(0));
    };
}

if (! Array.prototype.folded) {
    Array.prototype.folded = function (func, i) {
        var helper = function (arry) {
            if (arry && arry.length > 0) {
                i = func(i, arry.shift());
                return helper(arry);
            }
            return i;
        };

        return helper(this.slice(0));
    };
}


var a = [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ];

var b = a.filter(function (i) {
    return (i % 2 === 0) ? 1 : 0;
});

var c = a.map(function (i) {
    return i * 2;
});

var d = a.folded(function (n1, n2) {
    return n1 + n2;
}, 0);

var e = a.folded(function (n1, n2) {
    return n1 * n2;
}, 1);

console.log(a);

console.log(b); // filter
console.log(c); // map
console.log(d); // fold: sum
console.log(e); // fold: prod

console.log(a);

console.log([ [ 1, 2 ], [ 3, 4 ], [ 5, 6 ] ].folded(function (a, b) {
    return a.concat(b);
}, []);
// [ 1, 2, 3, 4, 5, 6 ]

配列（Array）のメソッド内部で使われてる helper 関数が使い捨て関数になってま。