Add arrow head to end of line or vector.

makeArrow.js

/* Thanks to sample code here:

https://github.com/wodory/flowmate
https://github.com/turbobabr/Sketch-Plugins-Cookbook

*/

var selection = context.selection;
var layer = selection.firstObject();
if(layer && layer.isKindOfClass(MSShapeGroup)) {
    var path = layer.layers().firstObject();
    if(path && path.isKindOfClass(MSShapePathLayer)) {
        path.setEndDecorationType(2);
    }
}