Debugging Event Handlers

Finding errors in code can be very hard, and doing so without any clues is very very hard. Unfortunately that's the potential problem facing everyone using the new installable event handlers.  The scripts that we write to handle events aren't initiated directly by a user at a screen, so there's no immediate place to report error messages and problems.  As a result we're frequently baffled as to why our scripts don't work as expected!

Here's a simple trick for catching errors and emailing them to yourself.  We're relying on a very nice feature of JavaScript called Exception Handling, and you can learn more about it here.  We take the existing event handler:

function myFunc(){
  // put your event handling code here.
}

and simply wrap it in a try/catch block:

function myFunc(){
  try{
    // put your event handling code here.
  } catch(error){
    MailApp.sendEmail("me@example.com", 
                      "Error Handling Event!", 
                      "Error: " + error);
  }     
}

Now every time your Event Handling code breaks, you'll get an email telling you all about it.

Posted by: Evin Levey, Google Apps Script Product Manager