You can get atan2 functionality by using both arguments in the ATan() function in JSL. If you're following the definition here (atan2 - Wikipedia, the free encyclopedia), the Wikipedia arguments y and x correspond to the x1 and x2 arguments, respectively, in the JSL function.
In general, you can find information on JSL functions by using the Scripting Index (found in the Help menu). The Scripting Index has entries for JSL functions as well as messages that can be sent via scripting to platform objects and display objects.
Michael Crotty
Principal Statistical Writer
Manager, Statistical Documentation
JMP Development
