Google Apps Script utility functions for persistent logging. See https://mogsdad.wordpress.com/?p=193.

Logger-AddonUtilities.gs

/** * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * AddonUtilities.gs
 *
 * A collection of general Google-Apps-Script utilities that are useful for
 * development of Add-ons.
 * 
 * From: gist.github.com/rubenrivera/89c16e85c2bb8556f95788d630dc4b7f
 * Based on: gist.github.com/mogsdad/39db2995989110537868
 *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

/**
 * Add-ons must run onOpen() without requiring authorization, so instead of enabling
 * BetterLog globally, it must be enabled specifically by code that is running outside
 * of AuthMode.NONE. This function is called in only those cases.
 *
 * Uses BetterLog library, MYB7yzedMbnJaMKECt6Sm7FLDhaBgl_dE
 *
 * From: gist.github.com/mogsdad/39db2995989110537868
 * 
 * @returns {Boolean}  Current logging state
 */
var betterLogStarted = false;  // Global to ensure we start just once per instance
var useSpreadsheet = false;    // Global to use or not a spreadsheet
var logSheetId = undefined;    // undefined => log to container sheet.
                               // Set to spreadsheet ID to log to external sheet.
function startBetterLog_() {
  if (!betterLogStarted) {
    if(useSpreadsheet){
      Logger = BetterLog.useSpreadsheet(logSheetId);
    } else {
      Logger = BetterLog;
    }
    betterLogStarted = true;
  }
  return betterLogStarted;
}

/**
 * Support client-side logs from HTML file javascript. First argument is
 * expected to be the name of a Logger method.
 * 
 * From: gist.github.com/rubenrivera/89c16e85c2bb8556f95788d630dc4b7f
 * Based on: gist.github.com/mogsdad/39db2995989110537868
 *
 * @param {object} arguments   All arguments from [1] are passed on
 */
function clientLog() {
  if (!startBetterLog_()) return;
  var args = Array.slice(arguments);    // Convert arguments to array
  var func = args.shift();              // Remove first argument, Logger method
  if (!Object.prototype.hasOwnProperty.call(Logger, func))     // Validate Logger method
    throw new Error( "Unknown Logger method: " + func );
  args[0] = "CLIENT "+args[0];          // Prepend CLIENT tag
  Logger[func].apply(null,args);        // Pass all arguments to Logger method
}