_chanIdByIdentifier (channel, identifier) {
    const channelIds = Object.keys(this._channelMap)
    let chan

    for (let i = 0; i < channelIds.length; i++) {
      chan = this._channelMap[channelIds[i]]

      if (chan.channel === channel && (
        chan.symbol === identifier ||
        chan.key === identifier
      )) {
        return channelIds[i]
      }
    }

    return null
  }
@param {string} channel - channel name
@param {string} identifier - unique identifier for the channel
@returns {number} channelID
@private