gistfile1.js

Object.prototype.setdefault = function(key, value) {
    if (this[key] === undefined) {
        this[key] = value;
    }
    return this[key];
};

test('Object.prototype.setdefault', function() {
        var o = { b: 2 };
        equals(1, o.setdefault('a', 1));
        equals(2, o.setdefault('b', 1));
        o.setdefault('c', []).push(3);
        equals(3, o.c[0]);
});