That is Javascript code which would run in the browser, or perhaps in the web server if it is implemented as a server-side script. Since the Arduino web server does not support server side scripting there's no way that's running on the Arduino. If it ran at the client (browser), it would create the file on the PC where the browser is running, not in the Arduino. If you want to create a file on the Arduino's SD card, your sketch must create the file.
If you want the existence of the file to trigger the Arduino to do something then it seems to me that it would be more sensible to trigger that directly at the point where the file is created and not go through the rigmarole of creating the file and then testing whether it existing later.