How to debug cinnamon applet?

Your best bet here is the old trick of printing messages to the terminal. Use global.log(...) for it, the lines are sent to the ~/.cinnamon/glass.log file, to watch them just do:

tail -f ~/.cinnamon/glass.log 

Also, as stated by @elParaguayo, type Alt+F2 to bring the Cinnamon prompt, type in lg and press ENTER to show the Looking Glass Debugger topmost dialog, there you can evaluate expressions by typing them in the Exec: input box and watching the results listed in the Results tab.

I know this is over a year old now, but I came across it as I was trying to write my own applet so I though I'd share my experiences.

In terms of debugging, I'm not sure these can be done in a browser. I debugged mine by updating my code, re-activating the applet in Cinnamon and then looking at the error log (Alt+F2 and then type lg).

Not a great solution at all, but it's the only solution I've found.

Use the browser is not possible. You need to read:

1. ~/.xsession-errors (because not all errors can appear as a cinnamon errors if you interact with other system applications) or you also can read the terminal output of cinnamon (same as xsession-errors, but will be update dynamically). Execute "cinnamon --replace" to enable the output mode, is possible that this will required to be execute more than one time.

2. ~/.cinnamon/glass.log or you can use Melange - Cinnamon Debugger.Just click on cinnamon panel, TroubleShoot and find the application.