shorten_number.js

var digits = '0123456789' +
  'abcdefghijklmnopqrstuvwxyz' +
  'ABCDEFGHIJKLMNOPQRSTUVWXYZ' +
  '-_';
var radix = digits.length;

function shorten_number(n) {
  var s = '';
  while (n > 0) {
    s = digits.charAt(n % radix) + s;
    n = Math.floor(n / radix);
  }
  return s || '0';
}