getAllLayersOfClass.js

var textLayers = getAllLayersOfClass("MSTextLayer"),
    textLayerLoop = textLayers.objectEnumerator(),
    textLayer;

while (textLayer = textLayerLoop.nextObject()) {
    log(textLayer.attributedStringValue());
}

function getAllLayersOfClass(className) {
	var array = NSArray.array(),
        pageLoop = MSDocument.currentDocument().pages().objectEnumerator(),
        page;

	while (page = pageLoop.nextObject()) {
		var predicate = NSPredicate.predicateWithFormat("className == %@",className),
		    matches = page.children().filteredArrayUsingPredicate(predicate);

		array = array.arrayByAddingObjectsFromArray(matches);
	}

	return array;
}