Hello,
The following minimal example using this js code
- command = "Reflect[A, mirror]";
ggbApplet.evalCommand(command);
fails to execute when hitting the button. mirror is a plane spanned by 3 points. A is some point (not included in the plane).
The error reads:
Using the inputline one can easily write 'Reflect[A,mirror]' and get the expected mirrored A'.
Since this totally messes up my expectiations how js is handled when using ggbApplet.evalCommand() I can't make any further suggestions how to solve this.
The same problem 
I`m feeling a bit stupid now... It actually does work for the point. and also for other objects (tested with plane since I will need it later on).with Mirror[]
Thanks for the fast answer. Though this is still strange because I don't get a Mirror[] command in autocompletion when typing it in the console. Yet it does work if I fully write it out...
Command Reflect:
Illegal Argument: Plane mirror
Syntax:
Reflect.syntax