Mass Rename script for AE

//General variables.
var theComp= app.project.activeItem;
var selectedLayers = theComp.selectedLayers;

//Starts mass-rename function.
massRename();

//Start of mass-rename function:
function massRename(){

    app.beginUndoGroup("layer rename");

    //Checks that the current item is a composition.
   if (!theComp || !(theComp instanceof CompItem)){
        alert("No composition is selected. Please select a composition and try again.");
        return;
    }

    //Checks that some layers are selected and sets the default name in the prompt to the bottom layer selected.
    var originalName = "placeholder";
    if(theComp.selectedLayers.length < 1){
        alert("No layers are selected.");
        return;
    }else{
        originalName = theComp.selectedLayers[0].name;
    }

    //Gets input for a new name from the user and checks that something actually has been inputted.
    var newName = prompt("Please enter a name", originalName);
    if(newName.length < 1){
        alert("No new name has been inputted.");
        return;
    }

    //Renames all the layers.
    for(var i =0; i < theComp.selectedLayers.length;i++){
        var name = newName + " " + String(i+1);
        var currLayer = theComp.selectedLayers[i];
        try{currLayer.source.name = name;}catch(error){ $.writeln( "The layer has no source."); }
        currLayer.name = name;
    }

    app.endUndoGroup();

}