Improvement 1: indication there is javascript error
Either the javascript validity is evaluated on a realtime basis (on “change” event on the text area) – and OK button should not be available till the syntax is validated (NB: this would be my implementation choice. It consumes more resource but not on a crucial moment), either it should be validated only when attempting to validate the function (which is currently the behaviour), but in this case, I would suggest to reset the error message immediately after the first change event on the text area (because it means the user has modified something, so the code could be good).
Improvement 2: indication what the error is
Javascript validation comes probably with an explanation error during validation. Please share it with the user, for accelerating the diagnosis and the correction!
Thanks