adds an iso week count to the js Date function

isoweek.js

/* getWeek returns an ISO 8601 week for any Date object.
 *
 * as far as i know, it handles all the weird boundary
 * conditions that come up, even by a millisecond.
 * no license, go nuts or whatever, or don't
 * --marcos@generic.cx
 */
Date.prototype.getWeek = function() {
  var fm = function(fullyear){
    // return a date for the first monday in a year
    var f = new Date(fullyear, 0, 1);
    var d = f.getDay();
    var m = (d > 4) ? 9 : 2;
    f.setDate(m - d); // shift from 1st to first mon
    return f;
  }
  var firstMon = fm(this.getFullYear());
  if ((this - firstMon) < 0){ // if date precedes the first monday
    firstMon = fm(this.getFullYear()-1);
  }
  return week = Math.floor(((this - firstMon) / 86400000)/7)+1;
}