Modified the gist to allow it to work with the front most document.

App icons.jstalk

function main(image, doc, layer) {
	var sizes = [29, 40, 50, 57, 58, 72, 76, 80, 100, 114, 120, 144, 152, 1024];
	 
	for (var idx = 0; idx < sizes.length; idx++) {
	    doc.scaleImageToWidth(sizes[idx]);
	    doc.dataRepresentationOfType("public.png").writeToFile("/Users/cotton/Desktop/icon-" + sizes[idx] + ".png");
	    doc.undo();
	}
}