makeAllStrokesRound.jsx

//Changes all selected Shape Layers stroke Join to Round
//CC-BY, Nik Ska, 2016

var makeAllStrokesRound = this;

makeAllStrokesRound.go = function(){
    var strokeName = "ADBE Vector Graphic - Stroke";

    var cycleGroups = function(_group){
        if(_group.matchName == strokeName){
            _group.property("ADBE Vector Stroke Line Join").setValue(2);
            _group.property("ADBE Vector Stroke Line Cap").setValue(2);
        }
        else{
            if(_group.numProperties > 0){
                for(var p = 1; p <= _group.numProperties; p++){
                    cycleGroups(_group.property(p));
                }
            }
        }
    }

    var activeComp = app.project.activeItem;
    if(activeComp && activeComp instanceof CompItem){
        var sel = activeComp.selectedLayers;
        if(sel.length > 0){
            app.beginUndoGroup("Making Strokes Round");
            for(var s = 0; s < sel.length; s++){
                if(sel[s] instanceof ShapeLayer){
                    cycleGroups(sel[s].property("ADBE Root Vectors Group"));
                }
            }
            app.endUndoGroup();
        }
    }
}

makeAllStrokesRound.go();