convertSelectionsToRect.js

var selections = context.selection,
	loop = selections.objectEnumerator(),
	selection;

while (selection = loop.nextObject()) {
	var rectangle = MSRectangleShape.new();

	rectangle.frame().x = selection.frame().x();
	rectangle.frame().y = selection.frame().y();
	rectangle.frame().width = selection.frame().width();
	rectangle.frame().height = selection.frame().height();

	var rectangleGroup = MSShapeGroup.shapeWithPath(rectangle);

	selection.parentForInsertingLayers().addLayers([rectangleGroup]);
	selection.removeFromParent();
}