Each parameter you define must be registered with JSAP via its registerParameter() method. When a parameter is registered, its properties (flags, etc.) cannot be modified; to modify them, you must first unregister them from JSAP, and register them again once modified. Registration can throw an exception if the parameter to be registered conflicts with any other registered parameters (for example, if they use the same flags, or have the same ID). If you're using SimpleJSAP, parameters passed as an argument to the constructor are registered for you.