Effective way to debug a Google Apps Script Web App

In Web Apps, even the most basic debugging of variables through Logger.log() does not work!

A great solution to have at least simple variable logging available is Peter Herrmann's BetterLog for Apps Script. It allows you to log into a spreadsheet (the same as your working spreadsheet or a separate one).

Installation is very simple - just add an external resource (see the Github readme) and a single line of code to override the standard Logger object:

Logger = BetterLog.useSpreadsheet('your-spreadsheet-key-goes-here');

Remember, that the spreedsheet that you give here as a parameter will be used for the logging output and thus must be writable by anybody!

BetterLog will create a new sheet called "Log" in the given spreadsheet and will write each log call into a separate row of that sheet.

In his excellent book "Google Script", James Ferreira advocates setting up your own development environment with three browser windows; one for the code, one for the live view (in Publish, Deploy as web app, you are provided with a "latest code" link that will update the live view to the latest save when it is refreshed), and one for a spreadsheet that logs errors (using try/catch wrapped around bits of code you want to keep an eye on).