abynim

// run this code in Sketch's script window (Plugins > Run Script...)
// tested in Sketch 57

context.document.currentPage().changeSelectionBySelectingLayers(nil);
let children = context.document.currentPage().children().objectEnumerator(), layer;
while(layer = children.nextObject()) {
    layer.select_byExtendingSelection(context.document.sidebarController().filterViewController().filter().shouldIncludeNode(layer), true);
}

abynim

// Name of layer to copy
var sourceLayerName = "gray_rectangle"

// Ask user to select a Sketch file:
var openDialog = NSOpenPanel.openPanel()
openDialog.setCanChooseFiles(true)
openDialog.setAllowedFileTypes(["sketch"])
openDialog.setCanChooseDirectories(false)
openDialog.setAllowsMultipleSelection(false)
openDialog.setCanCreateDirectories(false)

abynim

var doc = context.document

var selectLayersOfType_inContainer = function(layerType, containerLayer) {
    
    // Filter layers using NSPredicate
	var scope = (typeof containerLayer !== 'undefined') ? [containerLayer children] : [[doc currentPage] children],
		predicate = NSPredicate.predicateWithFormat("(className == %@)", layerType),
		layers = [scope filteredArrayUsingPredicate:predicate];
		
	// Deselect current selection

abynim

var writeTextToFile = function(text, filePath) {
    var t = [NSString stringWithFormat:@"%@", text],
    f = [NSString stringWithFormat:@"%@", filePath];
    return [t writeToFile:f atomically:true encoding:NSUTF8StringEncoding error:nil];
}

var readTextFromFile = function(filePath) {
    var fileManager = [NSFileManager defaultManager];
    if([fileManager fileExistsAtPath:filePath]) {
        return [NSString stringWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:nil];