start_of_week.js

Date.prototype.startOfWeek = function() {
  
  var dayOfMonth = this.getDate() - this.getDay();
  var month      = (dayOfMonth > 0) ? this.getMonth() : this.getMonth() - 1;
  var year       = (month > 0) ? this.getFullYear() : this.getFullYear() - 1;
  
  if (dayOfMonth < 0) {
    dayOfMonth = this.getDaysInMonth(month) - (dayOfMonth * -1)
  }
  
  return new Date(year, month, dayOfMonth,  this.getHours(), this.getMinutes(), this.getSeconds(), this.getMilliseconds());
  
};