Accessing the Currently-Selected Duration in a Plugin

This is a starting point:

// Retrieve the selection
var selection = curScore.selection;
var elements = selection.elements;
var all = [];
var prevChord = null;

// Keep only the CHORD and the RESTS elements.
// For the NOTE elements, retrieve their CHORD parent, and ensure it hasn't yet been added
for (var i = 0; i < elements.length; i++) {
    var element = elements[i];
    if (element.type === Element.NOTE) {
        // Got a note, need to find the parent chord
        var chord = element.parent;
        if (!prevChord || (prevChord !== chord)) {
            all.push(chord);
        }
        prevChord = chord;
    } else if ((element.type == Element.REST) || (element.type === Element.CHORD)) {

        all.push(element);

    }

}
console.log("found %1 elements".arg(all.length));

// Sum all the duration
// For the ease of the summuing, put all durations in x/64
var total = 0;
for (var j = 0; j < all.length; j++) {
    var duration = all[j].duration;
    var d = 64 * duration.numerator / duration.denominator;
    console.log("%1): %2 - %3".arg(j).arg(duration.str).arg(d));
    total = total + d;
}
console.log("total duration %1/%2".arg(total).arg(64));

I took a few assumptions (for the ease of the code readability):

• There are no tuplets: if there were tuplets, the .duration is the nominal element's duration. To get the effective (aka "real") duration, you must access the element's tuplet properties (its ratio).
• all the elements are on the same track. If multiple tracks were involved, you probably have to work by tracks and segments

Other approaches could go via the Cursor object:

var cursor = score.newCursor();

var firstTick, firstStaff, lastTick, lastStaff;
// start
cursor.rewind(Cursor.SELECTION_START);
firstTick = cursor.tick;
firstStaff = cursor.track;
// end
cursor.rewind(Cursor.SELECTION_END);
lastTick = cursor.tick;
if (lastTick == 0) { // dealing with some bug when selecting to end.
    lastTick = score.lastSegment.tick + 1;
}
lastStaff = cursor.track;
var chords = [];

cursor.rewind(Cursor.SELECTION_START);
var segment = cursor.segment;
while (segment && (segment.tick < lastTick)) {
    for (var track = firstStaff; track <= lastStaff; track++) {
        var element;
        element = segment.elementAt(track);
        if (element && (element.type == Element.CHORD || element.type == Element.REST)) {
            debugSegment(segment, track);
            chords[chords.length] = element;
        }
    }
    // cursor.next();
    // segment = cursor.segment;
    segment = segment.next; // 18/6/22 : looping thru all segments, and only defined at the curso level (which is bound to track)
}

Check which one works the best for your case in different use cases (in terms of selection), such as :