Select all items on the entire artboard and delete

give this a shot. I'm assuming you mean you want to delete everything in the document. If you only want to delete stuff that's on the artboard, let me know and we can work that in as well. This will ensure that everything is unlocked and visible, and will then delete all pageItems per @femkeblanco's suggestion of the removeAll() method. If you want to leave stuff that's locked/hidden alone, then you can use femkeblanco's second option as it is.

#target Illustrator
//Description: reveal all pageItems and layers, then delete all pageItems
function test()
{
	var docRef = app.activeDocument;
	var layers = docRef.layers;

	for(var l=0,len=layers.length;l<len;l++)
	{
		layers[l].locked = false;
		layers[l].visible = true;
	}

	app.executeMenuCommand("unlockAll");
	app.executeMenuCommand("showAll");

	docRef.pageItems.removeAll();
	
}
test();

wouldnt it be easier to use an action? The script above - will it select locked items? you can create an action that will do it all.

To select all items:

//select all items, by selecting each item individually, thru a loop
for (i = 0; i < activeDocument.pageItems.length; i++) {
  activeDocument.pageItems[i].selected = true;
}

To delete all items, without needing to select them:

//delete all items
activeDocument.pageItems.removeAll()

I use CS6.  I don't know if this works in CC.  For example, I don't need to make app. explicit.