gistfile1.js

Array.prototype.join2 = function(f) {
    if(typeof f != 'function') {
        return this.join(f);
    }

    return this.reduce(function(memo, item) {
        return memo + f() + item;
    });
};

["a", "b", "c"].join2(function() {
    var separator = ",.!#$%&";
    return separator[Math.floor(Math.random() * separator.length)];
});