JavaScript Firebase UID Generation

uid.ts

export interface IUID {
  generate(): string;
  timestamp(uid: string, asDate?: boolean): number | Date;
}

/**
 * SEE --> https://gist.github.com/mikelehen/3596a30bd69384624c11
 * 
 * Generator that creates 20-character string identifiers with the following properties:
 *
 * 1. They're based on timestamp so that they sort *after* any existing ids.
 * 2. They contain 72-bits of random data after the timestamp so that IDs won't collide.
 * 3. They sort *lexicographically* (timestamp converted to chars which will sort)
 * 4. They're monotonically increasing. This is done by storing previous random bits
 *    increasing them only by one.
 */

// ASCII ordered base64 web safe chars.
const ASCII_CHARS = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz';

export const uid: IUID = {

  /**
   * Generate
   * Generates uid push identifier.
   */
  generate: (function () {

    // Timestamp of last push, used to prevent local
    // collisions if you push twice in one ms.
    let lastPushTime = 0;

    // We generate 72-bits of randomness which get
    // turned into 12 characters and appended to the
    // timestamp to prevent collisions with other clients.
    // We store the last characters we generated because
    // in the event of a collision, we'll use those same
    // characters except "incremented" by one.
    const lastRandChars = [];

    return function () {

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

      lastPushTime = now;

      let timeStampChars: string[] = new Array(8);

      let i: number;

      for (i = 7; i >= 0; i--) {

        timeStampChars[i] = PUSH_CHARS.charAt(now % 64);

        // NOTE: Can't use << here because javascript
        // will convert to int and lose the upper bits.
        now = Math.floor(now / 64);

      }

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

      let id = timeStampChars.join('');

      if (!duplicateTime) {

        for (i = 0; i < 12; i++) {

          lastRandChars[i] = Math.floor(Math.random() * 64);

        }

      }

      // If the timestamp hasn't changed since
      // last push, use the same random number,
      // except incremented by 1.
      else {

        for (i = 11; i >= 0 && lastRandChars[i] === 63; i--) {

          lastRandChars[i] = 0;

        }

        lastRandChars[i]++;

      }

      for (i = 0; i < 12; i++) {

        id += ASCII_CHARS.charAt(lastRandChars[i]);

      }

      if (id.length !== 20)
        throw new Error('Length should be 20.');

      return id;

    };

  })(),

  /**
   * Timestamp
   * Gets timestamp from provided uid.
   */
  timestamp: (function () {

    return function (id: string, asDate?: boolean): number | Date {

      let time = 0;
      const data = id.substr(0, 8);

      for (let i = 0; i < 8; i++) {

        time = time * 64 + ASCII_CHARS.indexOf(data[i]);

      }

      if (asDate)
        return new Date(time);

      return time;

    };

  })()

};

I think you still need to change PUSH_CHARS into ASCII_CHARS