Math.(min|max) で配列のmax, minメソッドの実装

gistfile1.js

var ap = Array.prototype;
if (! ap.forEach_) {
    ap.forEach_ = function (f) {
        for (var i = 0, len = this.length; i < len; i++) {
            f(this[i], i);
        }
    };
}

if (! ap.grep) {
    ap.grep = function (f) {
        var that = [];
        this.forEach_(function (a) {
            if (f(a)) that.push(a);
        });
        return that;
    };
}

if (! ap.max) {
    ap.max = function () {
        return Math.max.apply(null, this.grep(function (n) {
            return (n === n - 0) ? true : false;
        }));
    };
}
if (! ap.min) {
    ap.min = function () {
        return Math.min.apply(null, this.grep(function (n) {
            return (n === n - 0) ? true : false;
        }));
    };
}

var a = [ 'abc', 0, -23, 4, 3, -2, 9, '12' ];

console.log(a.max()); // 9
console.log(a.min()); // -23