Function.prototype.method = function (name, func) {
    this.prototype[name] = func;
    return this;
};

Number.method('integer', function () {
    return Math[this < 0 ? 'ceil' : 'floor'](this);
});

document.writeln((-10 / 3).integer()); // -3

var num = Number('1.2');
alert(num instanceof Number); // true
alert(num instanceof Function); // false
alert(Number instanceof Number); // false
alert(Number instanceof Function); // true