Is it possible to automatically determine my current target selection using only one variable?

@dublove this is one way to do it:

/**
 * Example of starting a script by getting the
 * selected stories or the whole document, but first
 * confirming whether to process the whole document.
 * @author m1b
 * @version 2025-03-25
 * @discussion https://community.adobe.com/t5/indesign-discussions/is-it-possible-to-automatically-determine-my-current-target-selection-using-only-one-variable/m-p/15227968
 */
(function () {

    var targets = getStoriesOrDocument(app.activeDocument);

    if ('Document' === targets.constructor.name) {

        if (!confirm('Do you want to apply to whole document?'))
            return;

        // store document in an array for processing
        targets = [targets];

    }

    // now do something with the stories or document
    for (var i = 0; i < targets.length; i++) {
        var target = targets[i];
        $.writeln(target.constructor.name + ' ' + target.id);
    }

})();

/**
 * Returns the document's selected
 * stories or the document itself.
 * @author m1b
 * @version 2025-03-25
 * @param {Document} doc - an Indesign Document.
 * @returns {Array<Story>|Document}
 */
function getStoriesOrDocument(doc) {

    var stories = [],
        unique = {},
        items = doc.selection;

    if (0 === items.length)
        return doc;

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

        if (items[i].hasOwnProperty('parentStory')) {

            if (unique[items[i].parentStory.id])
                // already collected this story
                continue;

            unique[items[i].parentStory.id] = true;
            items[i] = items[i].parentStory;
        }

        if ('function' === typeof items[i].changeGrep)
            stories.push(items[i]);

    }

    return stories;

};

Edit 2025-03-25: added support for other selections, eg. an InsertionPoint.