Date.prototype.getNextWeekMonday = function() {
    var d = new Date(this.getTime());
    var diff = d.getDate() - d.getDay() + 1;
    if (d.getDay() == 0)
        diff -= 7;
    diff += 7; // ugly hack to get next monday instead of current one
    return new Date(d.setDate(diff));
};

Date.prototype.getNextWeekFriday = function() {
    var d = this.getNextWeekMonday();
    return new Date(d.setDate(d.getDate() + 4));
};

// You can then use it like this :

var date = new Date();
alert(date.getNextWeekMonday());
alert(date.getNextWeekFriday());