Script to convert shape rounded rectangle to rectangle

If you wish to create or modify a rectangle with rounded corners, you will have to add/remove the extra path points and set their Bezier handles yourself.

https://ai-scripting.docsforadobe.dev/jsobjref/PathPoints.html

That is how the corner rounding options in Properties panel really does it, which you can check:

alert(app.activeDocument.pathItems[0].pathPoints.length)