Script to arrange one pathItem per layer

Again, there are so many ways, but here is one approach. I wanted to do it differently to the first script to make it more flexible. This time you can specify a targetLayerName (or names if you use an array, or none to apply to all layers). Like this:

/**
 * @File One Item Per Layer 3.js
 *
 * Moves each of a layer's page items to a new top-level layer,
 * matching properties of the item's original layer.
 *
 * @7111211 m1b
 * @version 2024-12-21
 * @discussion https://community.adobe.com/t5/illustrator-discussions/script-to-arrange-one-pathitem-per-layer/m-p/15047388
 */
(function () {

    var settings = {

        /**
         * The layer(s) to target
         *   - if String, target that single layer
         *   - if Array, target multiple layers
         *   - if undefined, target all layers
         */
        targetLayerName: ['Sublayer 2.1', 'Sublayer 2.1.1'],

    };

    if (0 === app.documents)
        return alert('Please open a document and try again.');

    if (
        undefined != settings.targetName
        && 'Array' !== settings.targetName.constructor.name
    )
        settings.targetName = [settings.targetName];

    var doc = app.activeDocument,
        items = doc.pageItems,
        moves = [],
        removeMe = new Unique();

    itemLoop:
    for (var i = 0; i < items.length; i++) {

        var item = items[i];

        if (!item.hasOwnProperty('parent'))
            continue;

        var obj = item,
            topLayer = undefined,
            parentLayer = undefined;

        while (!topLayer) {

            obj = obj.parent;

            if (!obj.hasOwnProperty('parent'))
                continue itemLoop;

            if (
                'Layer' === obj.constructor.name
                && !parentLayer
            )
                parentLayer = obj;

            if ('Document' === obj.parent.constructor.name)
                topLayer = obj;

        }

        if ('Layer' !== topLayer.constructor.name)
            continue itemLoop;

        if (settings.targetLayerName) {

            if (indexOf(parentLayer.name, settings.targetLayerName) !== -1)
                // match this layer by name
                removeMe.push(parentLayer);
            else
                continue itemLoop;
        }
        else
            removeMe.push(topLayer);

        // remove the layer once empty
        removeMe.push(topLayer);

        moves.push({
            item: items[i],
            layer: topLayer,
            layerName: parentLayer.name,
        });

    }

    // perform the moves
    for (var i = moves.length - 1; i >= 0; i--) {

        var move = moves[i],
            newLayer;

        move.layer.locked = false;
        move.layer.visible = true;

        // create the new layer
        newLayer = doc.layers.add();
        newLayer.color = move.layer.color;
        newLayer.locked = move.layer.locked;
        newLayer.name = move.layerName;
        newLayer.preview = move.layer.preview;
        newLayer.printable = move.layer.printable;
        newLayer.visible = move.layer.visible;

        // move into place
        newLayer.move(move.layer, ElementPlacement.PLACEAFTER);
        move.item.move(newLayer, ElementPlacement.PLACEATBEGINNING);

    }

    // remove empty layers
    for (var i = removeMe.length - 1; i >= 0; i--)
        if (isEmpty(removeMe[i]))
            removeMe[i].remove();

})();

/**
 * An array-like object that won't contain duplicates.
 * @7111211 m1b
 * @version 2024-12-21
 */
function Unique() {

    this.array = [];
    this.length = 0;

    this.push = function pushUnique(obj) {

        for (var i = 0; i < this.array.length; i++)
            if (this.array[i] === obj)
                return;

        this.array.push(obj);
        this.length++;
        this[this.length - 1] = obj;

    };

    this.toString = function uniqueToString() {
        return String(this.array);
    };

};

/**
 * Returns index of obj in arr.
 * Returns -1 if not found.
 * @9397041 {any} obj
 * @9397041 {Array} arr
 * @Returns {Number}
 */
function indexOf(obj, arr) {
    for (var i = 0; i < arr.length; i++)
        if (arr[i] === obj)
            return i;
    return -1;
};

/**
 * Returns true when the layer and its sublayers are all empty.
 * @9397041 {Layer} layer - an Illsutrator Layer.
 * @Returns {Boolean}
 */
function isEmpty(layer) {

    if (!layer)
        return false;

    if (layer.layers.length > 0) {
        for (var i = 0; i < layer.layers.length; i++) {
            if (!isEmpty(layer.layers[i]))
                return false;
        }
    }

    if (layer.pageItems.length > 0)
        return false;

    return true;

};