移譲を使う（いい例じゃない）

gistfile1.js

var t1, add;

t1 = function () {
    var Hold = function (_num) {
        this.num = _num;
        this.add = function (num2) {
            return toNumber(this.num) + toNumber(num2);
        };
    };

    var hold = new Hold (3);
    test(hold.add(4), 7);

    add = function (_num, num2) {
        return hold.add.apply({ num : _num }, [ num2 ]);
    };
};


t1();
test(add(8, 9), 17);

/* Functions */

function test (res_func, res) {
    var is_success = (res_func === res) ? 'success:' : '!  faild:';
    console.log([is_success,res_func, res].join(" "));
}

function toNumber (n) {
    return (typeof n === 'number') ? n
         : (Number(n))             ? Number(n)
         :                           0;
}