There seems to be a subtle bug: data.listeners++ will fail the first time [Edit: sorry, false alarm]

[teetotum]

I'm reading the source code to understand how it works and I think I found a subtle bug.
data.listeners++ will fail the first time with a runtime error because data is undefined.
I haven't verified it during runtime yet. And it's in the middle of the night here and I need to sleep. Will look at it again tomorrow; but thought it wise to report this here lest I forget.

function onAddListener(originalFn, type, handler, options) {
    // Polyfill scrollend event on any element for which the developer listens
    // to 'scrollend' explicitly or 'scroll' (so that adding a scrollend listener
    // from within a scroll listener works).
    if (type != "scroll" && type != "scrollend") return;

    let scrollport = this;
    let data = observed.get(scrollport);
    if (data === undefined) {
      let timeout = 0;
      data = {
        scrollListener: (evt) => {
          clearTimeout(timeout);
          timeout = setTimeout(() => {
            if (pointers.size) {
              // if pointer(s) are down, wait longer
              setTimeout(data.scrollListener, 100);
            } else {
              // dispatch
              if (scrollport) {
                scrollport.dispatchEvent(scrollendEvent);
              }
              timeout = 0;
            }
          }, 100);
        },
        listeners: 0, // Count of number of listeners.
      };
      originalFn.apply(scrollport, ["scroll", data.scrollListener]);
      observed.set(scrollport, data);
    }
    data.listeners++;
  }

EDIT: I was wrong.

As promised I looked at it by daylight and see now that the data variable will be initialized within the if (data === undefined) {...} branch. So no bug there. That's why I didn't open an issue in the night; only a question.