resubscribePreviousChannels () {
    Object.values(this._prevChannelMap).forEach((chan) => {
      const { channel } = chan

      switch (channel) {
        case 'ticker': {
          const { symbol } = chan
          this.subscribeTicker(symbol)
          break
        }

        case 'trades': {
          const { symbol } = chan
          this.subscribeTrades(symbol)
          break
        }

        case 'book': {
          const { symbol, len, prec } = chan
          this.subscribeOrderBook(symbol, prec, len)
          break
        }

        case 'candles': {
          const { key } = chan
          this.subscribeCandles(key)
          break
        }

        default: {
          debug('unknown previously subscribed channel type: %s', channel)
        }
      }
    })
  }
Subscribes to previously subscribed channels, used after reconnecting

@private