File content rollover

Here's what I implemented. I'll leave it here for further reference.
The full code can be found at https://github.com/msdeibel/BangleApps/blob/master/apps/batchart/widget.js

function logBatteryData() {
      const previousWriteLogName = "bcprvday";
      const previousWriteDay = require("Storage").read(previousWriteLogName);
      const currentWriteDay = new Date().getDay();

      const logFileName = "bclog" + currentWriteDay;

      // Change log target on day change
      if (previousWriteDay != currentWriteDay) {
        //Remove a log file containing data from a week ago
        require("Storage").erase(logFileName);
        require("Storage").write(previousWriteLogName, currentWriteDay);
      }

      var bcLogFileA = require("Storage").open(logFileName, "a");
      if (bcLogFileA) {
        console.log([getTime().toFixed(0), E.getBattery(), E.getTemperature(), getEnabledConsumersValue()].join(","));
        bcLogFileA.write([[getTime().toFixed(0), E.getBattery(), E.getTemperature(), getEnabledConsumersValue()].join(",")].join(",")+"\n");
      }
  }