How to turn selected layers into symbols？

This feature is not available when using the old version of an，It can only be solved with this command，but having trouble with the code

var symbolName = prompt("Enter new graphic name");

if(symbolName != null) {
		
	var version = fl.version;
	var versionNum = parseInt(version.substring(version.indexOf(" "), version.indexOf(",")));
	
	var dom = fl.getDocumentDOM(); 
	var tl = dom.getTimeline(); 
	
	//create rectangle at top (guide for stage)
	var guideLayerNum = tl.addNewLayer('_stageGuide');
	tl.setSelectedLayers(guideLayerNum, true);
	
	var stroke = dom.getCustomStroke('toolbar');
	var strokeCopy = dom.getCustomStroke('toolbar');
	var fill = dom.getCustomFill('toolbar');
	var useCustomStroke = new Boolean();
	useCustomStroke = stroke.style == "noStroke" && fill.style == "noFill";
	if(useCustomStroke) { 
		//fillCopy.color = 0xffffff; //attempting to set this var crashes flash
		stroke.style = 'solid';
		stroke.thickness = 1;
		stroke.color = 0xff0000;	//deprecated as of CS3
		//stroke.shapeFill.color = 0xff0000; //(for next version of Flash)
		if(versionNum >= 😎 stroke.scaleType ='none'; //flash 8 & above
		dom.setCustomStroke(stroke);
	}
	var w = parseInt(dom.width);
	var h = parseInt(dom.height);
	dom.addNewRectangle({left:0,top:0,right:w,bottom:h},0, false, false);
	tl.setLayerProperty('locked', true);
	tl.reorderLayer(guideLayerNum, 0, true);
	
	var frameSeq = tl.getSelectedFrames(); 
	var layerSeq = tl.getSelectedLayers(); 
	var lib = dom.library; 
	
	//clear [user] selected layers
	var selLayers = tl.getSelectedLayers();
	for(var i = 0; i < selLayers.length; i++){
		tl.setSelectedLayers(selLayers[i], true);
	}
	
	//clear [user] selected frames
	var clearFrameSel = new Array();
	for (var i = 0; i < frameSeq.length; i+=3){
		var tlayer = frameSeq[i];
		clearFrameSel.push(tlayer, -1, -1);
		tl.currentLayer = tlayer;
		tl.setSelectedLayers(tlayer, true);
	}
	
	tl.setSelectedFrames(clearFrameSel, true);
	var layerProps = new Array();
	
	//select all frames in timeline
	var endFrameArr = new Array();
	var frameSel = new Array();
	tl.expandFolder(true, true, -1); //expands all folders
	for (var i = 0; i < tl.layerCount; i++){
		var tlayer = tl.layers[i];
		var props = new Object();
		props.locked = tlayer.locked;
		props.visible = tlayer.visible;
		layerProps.push(props);
		frameSel.push(i, 0, tl.frameCount);
		endFrameArr.push(tlayer.frameCount);
		tl.currentLayer = tlayer;
		tl.setSelectedLayers(i, i==0);
	}
	
	tl.setSelectedFrames(frameSel, true);
	tl.copyFrames();
	
	//delete guide rectangle
	//tl.deleteLayer(0);
	

	if(symbolName == symbolName == "" || symbolName == undefined || symbolName.length < 1) symbolName = "_stageClip";
	
	lib.addNewItem('graphic', symbolName, 'top left');
	
	//dom.enterEditMode();
	lib.editItem(symbolName);
	var newTL = dom.getTimeline();
	newTL.selectAllFrames();
	newTL.pasteFrames();
	
	//add end frames & set layer props
	for (var i = 0; i < newTL.layerCount; i++){
		if(endFrameArr[i] < newTL.frameCount){
			newTL.setSelectedLayers(i, true);
			newTL.removeFrames(endFrameArr[i], newTL.frameCount);
		}
		newTL.layers[i].locked = layerProps[i].locked;
		newTL.layers[i].visible = layerProps[i].visible;
	}
	
	//set guide rectangle to outlines
	newTL.setSelectedLayers(0, true);
	newTL.setLayerProperty('outline', true);
	newTL.setLayerProperty('layerType',  'guide');
	
	//reset stroke color
	//if(useCustomStroke) {
		/*fl.trace('attempting to reset stroke');
		strokeCopy.style = "noStroke";
		strokeCopy.color = null;
		dom.setCustomStroke(strokeCopy);*/ //won't reset to null for some reason
	//}
	
	//return to main timeline
	dom.exitEditMode();
	tl = dom.getTimeline();

	for(var i=tl.layerCount; i > 0; i--){
		tl.currentLayer = i;
		tl.deleteLayer();
	}
	tl.currentLayer = 0;
	tl.setLayerProperty('locked', false);
	tl.setLayerProperty('name', symbolName);
	tl.selectAllFrames();
	tl.clearKeyframes();
	
	var newLayerNum = 0;
	lib.addItemToDocument({x:0, y:0}, symbolName);
	tl.insertFrames(newTL.frameCount);
	tl.layers[newLayerNum].frames[0].elements[0].symbolType = "graphic";
	tl.layers[newLayerNum].frames[0].elements[0].firstFrame = 0;
	tl.layers[newLayerNum].frames[0].elements[0].loop = "play once";
	tl.layers[newLayerNum].frames[0].elements[0].x = dom.width/2;
	tl.layers[newLayerNum].frames[0].elements[0].y = dom.height/2;

}