No. First map then sort.

maybe i will just not sort and just leave it as the original sorting being used.

?

let torrents = this.controller._rows
   .map(row => row.getTorrent())
   .sort((a, b) => b.getDateAdded() - a.getDateAdded());

if _rows are already being sorted by the controller then toSorted is unnecessary i think. see my other comment below.

Yes, in JavaScript, chaining .map() and .toSorted() does create two intermediate arrays — one after .map(), and another after .toSorted().

I figured you needed the last added X torrents to get the last X used paths. Are you sure the controller has the row/torrents in the correct order already?

if they have the sorting set to Age then yes

Both Set.has and Set.add traverse the entire set to check if an item is in the collection. You should change Set to Array since you are checking the collection yourself anyway.

or just avoid the has() call?

if (dir)
  dirs.add(dir)

or just avoid the has() call?

if (dir)
  dirs.add(dir)

Unfortunately not. There has to be some check to skip the loop if the dir already exists to avoid duplicate option elements.

afaik Set.has is O(1) while Array.includes is O(n)

A quick search led me to this article: Using Array.prototype.includes() vs Set.prototype.has() to filter arrays

And the comments painted a different picture from the main article: It appears to be implementation dependent.

If you want to be sure you can do some online Javascript performance testing.

from the article's comments there's a sample benchmark code and this is what i got:

in our case perf may be not noticeable at all but i think it is still good practice to use Set.

Ok. Then we can simplify(?) to:

workarea.append((() => {
  const datalist = document.createElement('datalist');
  datalist.id = 'add-dialog-folder-datalist';

  var i = 0;
  for (const dir of new Set(this.controller._rows.map(row => row.getTorrent().getDownloadDir().trim()))) {
    const option = document.createElement('option');
    option.value = dir;
    datalist.append(option);

    if (++i >= max_datalist_options) { // do not explode the ui
      break;
    }
  }
  return datalist;
})());

IF an option is created with a null and/or "" it should not appear in the input because you have to type something.

I think by using map() we could very well be using reduce(), it is accumulator for new Set() and do less job when reaching 100

    const datalist = document.createElement('DATALIST');
    datalist.id = 'add-dialog-folder-datalist';
    const dirs = this.controller._rows.reduce(
      (accumulator, row) => {
        if (accumulator.size < max_datalist_options) {
          accumulator.add(row.getTorrent().getDownloadDir().trim());
        }
        return accumulator;
      },
      new Set(),
    );
    for (const dir of dirs) {
      datalist.append(new Option(dir));
    }

    workarea.append(input, datalist);

Performance untested