Order Id generator algorithm for Firebase

i want a unique and readable id for the orders/invoices generated on firebase with the cloud functions. I think that a random id based on the uid and timestamp is a good solution because a user cannot send two orders at the same time.

What do you think?

i have edited this gist => http://ift.tt/1E1nSWn

and this is the results:

export default (() => {
  const PUSH_CHARS = '0123456789abcdefghijklmnopqrstuvwxyz';

  let lastPushTime = 0;
  let lastRandChars = [];

  return (uid) => {
    if (uid.length != 28) throw new Error('UID length should be 28.');

    let now = new Date().getTime();
    const duplicateTime = (now === lastPushTime);
    lastPushTime = now;

    // CONVERT TimeStamp to CHARS
    const timeStampChars = new Array(8);
    let i;
    for (i = timeStampChars.length - 1; i >= 0; i--) {
      timeStampChars[i] = PUSH_CHARS.charAt(now % 36);

      now = Math.floor(now / 36);
    }
    if (now !== 0) throw new Error('We should have converted the entire timestamp.');

    let id = timeStampChars.join('');

    // ADD 2 random CHARS
    for (i = 0; i < 2; i++) {
      id += PUSH_CHARS.charAt(Math.floor(Math.random() * 36)).toLowerCase();
    }
    Math.max

    // ADD random chars of UID
    if (!duplicateTime) {
      for (i = 0; i < 6; i++) {
        lastRandChars[i] = Math.floor(Math.random() * 28);
      }
    } else {
      for (i = 5; i >= 0 && lastRandChars[i] === 27; i--) {
        lastRandChars[i] = 0;
      }
      lastRandChars[i]++;
    }
    for (i = 0; i < 6; i++) {
      id += uid.charAt(lastRandChars[i]).toLowerCase();
    }

    // The id must be 16
    if (id.length != 16) throw new Error('Length should be 16.');

    return id;
  };
})();