Storing Compositions and Running Functions on them

Question

Firsty, I apologize for posting 2 posts in a row on this forum. I am having issues with a few things. I am trying to find a way to scan or store any compositions in a project so that they can be selected in a dropdown. Whatever they select in the dropdown will be the Comp that my mainFunction applies to. I have tried a few things, such as running a for loop to go through each project item and check if it's a composition. I am having struggles with this and there's probably some easy way to do it that I'm overlooking.
var numItems = app.project.numItems;
var allComps = new Array();
for (i = 1; i < numItems; i++) {
        if (app.project.items[i].label = 15){
        allComps.push(app.project.items[i].name);
var mainWindow = new Window("palette", "INSERT PROGRAM NAME", undefined);
mainWindow.orientation = "row";
var groupOne = mainWindow.add("group", undefined, "GroupOne");
groupOne.orientation = "column";
var entryComp = groupOne.add("dropdownlist", undefined, allComps);
var panelOne = mainWindow.add("panel", undefined, "Options");
var startButton = panelOne.add("button", undefined, "Start");
startButton.onClick = function(){
    app.beginUndoGroup("Undo");
    var thisComp = entryComp.selection.name;
    var midLayer = thisComp.layers[2];
    mainFunction();
function mainFunction() {
    precomposeTimer ();
mainWindow.center();
mainWindow.show

Answer 1

If your script allows it, you can simply ask the user to select the comp.
It doesn't cost more effort than selected something in a dropdown, and is easier to handle, allows smaller UI etc.
Otherwise you can store the comp objects into an array as David showed, and refer to them by their index in the  allComps array, not by their own index, nor name.
But then you'll need a reload button, with an onClick that does the same thing again.
Xavier.
var allComps=[], thisComp, midLayer;
// + code for the UI + a reload button
reloadButton.onClick =function(){
    var k;
    while (entryComp.items.length) entryComp.remove(entryComp.items[0]);
    allComps = getAllComps();
    for (k=0; k<allComps.length; k++) entryComp.add("item", allComps[k].name);
    // this leaves the dropdown with a null selection
    // it is possible to refine though
    return;
startButton.onClick = function(){
    if (entryComp.selection === null)  {reloadButton.notify(); return;};
    thisComp = allComps[entryComp.selection.index];
    if (Object.isValid(thisComp)){
        if (thisComp.numLayers<2){
            alert("layer 2 is missing...");
        else{
            midLayer = thisComp.layers[2];
            app.beginUndoGroup("Undo");
            mainFunction();
            app.endUndoGroup();
    else{
        reloadButton.notify();
    return;
reloadButton.notify();