Short 'hash' ID generator.

hashid.js

'use strict';

/**
 * The default alphabet is 25 numbers and lowercase letters.
 * Any numbers that look like letters and vice versa are removed:
 * 1 l, 0 o.
 * Also the following letters are not present, to prevent any
 * expletives: cfhistu
 */
var ALPHABET =
  '23456789abdegjkmnpqrvwxyz';

var ALPHABET_LENGTH = ALPHABET.length;

// Governs the length of the ID.
// With an alphabet of 25 chars,
// a length of 8 gives us 25^8 or
// 152,587,890,625 possibilities.
// Should be enough...
var ID_LENGTH = 8;

/**
 * Governs the number of times we should try to find
 * a unique value before giving up.
 * @type {Number}
 */
var UNIQUE_RETRIES = 9999;

var HashID = {};

/**
 * Returns a randomly-generated friendly ID.
 * Note that the friendly ID is not guaranteed to be
 * unique to any other ID generated by this same method,
 * so it is up to you to check for uniqueness.
 * @return {String} friendly ID.
 */
HashID.generate = function() {
  var rtn = '';
  for (var i = 0; i < ID_LENGTH; i++) {
    rtn += ALPHABET.charAt(Math.floor(Math.random() * ALPHABET_LENGTH));
  }
  return rtn;
};

/**
 * Tries to generate a unique ID that is not defined in the
 * `previous` array.
 * @param  {Array} previous The list of previous ids to avoid.
 * @return {String} A unique ID, or `null` if one could not be generated.
 */
HashID.generateUnique = function(previous) {
  previous = previous || [];
  var retries = 0;
  var id;

  // Try to generate a unique ID,
  // i.e. one that isn't in the previous.
  while(!id && retries < UNIQUE_RETRIES) {
    id = HashID.generate();
    if(previous.indexOf(id) !== -1) {
      id = null;
      retries++;
    }
  }

  return id;
};

module.exports = HashID;

is this will generate unique even after node js server restarts?

No, for this purpose you have to store the IDs in some kind of database.
You can pass an array of previous IDs to this module.

Btw @fiznool: I've updated your gist for my needs. I needed a configurable length and alphabet at runtime, so I just passed a few options to your module. Defaults will be used if no options are provided.

https://gist.github.com/derMani/9db056cfd526b553abdc

Regards
derMani

Explanation for the code: http://fiznool.com/blog/2014/11/16/short-id-generation-in-javascript/

In your opinion, for a simpler case, would Math.random().ToString(36).substr(2, length) suffice?
Thank you!

Can you "dehash" a hashed string back to normal?