code: function (cb) {
            /**
             * This callback receives generated id
             * @callback callback
             * @param {Error} error - Error occurred during id generation
             * @param {Buffer} id - Generated id
             */
  
              var id = new Buffer(8), time = Date.now() - this.epoch;
              id.fill(0);
  
              // Generates id in the same millisecond as the previous id
              if (time === this.lastTime) {
  
                  // If all sequence values (4096 unique values including 0) have been used
                  // to generate ids in the current millisecond (overflow is true) wait till next millisecond
                  if (this.overflow) {
                      overflowCond(this, cb);
                      return;
                  }
  
                  // Increase sequence counter
                  /*jslint bitwise: true */
                  this.seq = (this.seq + 1) & this.seqMask;
  
                  // sequence counter exceeded its max value (4095)
                  // - set overflow flag and wait till next millisecond
                  if (this.seq === 0) {
                      this.overflow = true;
                      overflowCond(this, cb);
                      return;
                  }
              } else {
                  this.overflow = false;
                  this.seq = 0;
              }
              this.lastTime = time;
  
              id.writeUInt32BE(((time & 0x3) << 22) | this.id | this.seq, 4);
              id.writeUInt8(Math.floor(time / 4) & 0xFF, 4);
              id.writeUInt16BE(Math.floor(time / FlakeId.POW10) & 0xFFFF, 2);
              id.writeUInt16BE(Math.floor(time / FlakeId.POW26) & 0xFFFF, 0);
  
              if (cb) {
                  process.nextTick(cb.bind(null, null, id));
              }
              else {
                  return id;
              }
          }
docs: Generates conflice-free id
  @param {cb=} callback The callback that handles the response.
  @returns {Buffer} Generated id if callback is not provided
  @exception if a sequence exceeded its maximum value and a callback function is not provided