Validate a Swedish personnummer with Luhn's Algorithm

gistfile1.js

// Expects a Swedish personnummer formatted as YYYYMMDDXXXX
var isLuhn = function(pnr) {
  var multiplier = 1;
  var nums = pnr.substr(2).split('').map(function(numStr) {
    multiplier = 1 + (multiplier % 2);
    var num = parseInt(numStr) * multiplier;
    return (num < 10) ? num : num - 9;
  });

  return nums.reduce(function(prev, curr) { return curr + prev; }) % 10 === 0;
}