How to define a round corner textframe in Javascript

Why are you trying to make you life so hard? 😉 

Use Object Styles - when you need to reformat those objects - you won't have to go through them all again - just a quick edit of the Object Style - that can be even done by the end user without the need to bother you to make changes in the script 😉 

And if you have a need for a different corners for different types of objects - you can just apply a different Object Style. 

That will depend on the version of InDesign you are using.

There are three (or more) distinct ways to apply rounded corners:

1.a You could add a text frame and define cornerOption and cornerRadius (InDesign CS4)

1.b You could add a text frame and define bottomLeftCornerOption, bottomLeftCornerRadius etc. (InDesign CS5 and CS5.5)

2. You could use the method convertShape() on a rectangular text frame.

3. The geometric way involving circles (ovals) and rectangles and adding them to one single path.

4. Or build one path with rounded corners from scratch and change its contentType to ContentType.TEXT_TYPE

See Jongware's reference for the DOM objects and methods:
http://www.jongware.com/idjshelp.html

Uwe