get the week in year for a date use javascript

        Date.prototype.getWeek = function () {

            //first day in week is Sunday

            var firstDayOfYear = new Date(this.getFullYear(), 0, 1);

            var offset = firstDayOfYear.getDay();

            var newDate = this;

            newDate.setDate(this.getDate() + offset);

            return Math.floor((newDate - firstDayOfYear) / 86400000 / 7 + 1);

        }