ADS1115 analog gain signal module - how to run it with espruino?

ADS1X15.prototype.getADC = function(channelSpec, callback) {
  var config = CONFIG.CQUE_NONE    | // Disable the comparator (default val)
                    CONFIG.CLAT_NONLAT  | // Non-latching (default val)
                    CONFIG.CPOL_ACTVLOW | // Alert/Rdy active low   (default val)
                    CONFIG.CMODE_TRAD   | // Traditional comparator (default val)
                    CONFIG.DR_1600SPS   | // 1600 samples per second (default)
                    CONFIG.MODE_SINGLE;   // Single-shot mode (default)
  // single ended (channelSpec is a number) or differential (channelSpec is array w/ valid channel duo)
  if ("number" == typeof channelSpec) { // Set single-ended input channel
    config |= [CONFIG.MUX_SINGLE_0,CONFIG.MUX_SINGLE_1,CONFIG.MUX_SINGLE_2,CONFIG.MUX_SINGLE_3][channelSpec];
  } else { // Set differential input channels from channelSpec
    var dif = DIFFS[channelSpec];
    if (typeof dif === "undefined") throw new Error("Invalid differential channelSpec " + channelSpec);
    config |= dif;
  } 

Looking at the module code, I can't see how it's getting to that error with 0 as the channelSpec.... since "number"==typeof 0 should be true...