droppedEntriesCount documentation incorrect

[tunetheweb]

MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/PerformanceObserver/PerformanceObserver

What specific section or headline is this issue about?

https://developer.mozilla.org/en-US/docs/Web/API/PerformanceObserver/PerformanceObserver#droppedentriescount

What information was incorrect, unhelpful, or incomplete?

This page (added in #22318) says that droppedEntriesCount is an optional property of PerformanceObserverCallback but it's a nested property from within a PerformanceObserverCallbackOptions.

What did you expect to see?

PerformanceObserverCallback can return an optional [PerformanceObserverCallbackOptions](https://w3c.github.io/performance-timeline/#dom-performanceobservercallbackoptions) dictionary property which will contain droppedEntriesCount (when supported).

Do you have any supporting links, references, or citations?

The code sample is also wrong and should be like this:

function perfObserver(list, observer, options) {
  list.getEntries().forEach((entry) => {
    // do something with the entries
  });
  if (options?.droppedEntriesCount > 0) {
    console.warn(
      `${options?.droppedEntriesCount} entries got dropped due to the buffer being full.`,
    );
  }
}
const observer = new PerformanceObserver(perfObserver);
observer.observe({ type: "resource", buffered: true });

You can also see this in the spec: https://w3c.github.io/performance-timeline/#dom-performanceobservercallbackoptions

Do you have anything more you want to share?

I'd fix this myself, but seems a little complicated as to how PerformanceObserverCallback would be set up and got a little lost when trying it myself.

Also not sure whether bcd should be updated to nest droppedEntriesCount under PerformanceObserverCallbackOptions or if it's fine to leave where it is?

All in all, could do with someone more familiar with the structure of MDN to tackle this.

Happy to help review any changes here though!

[jasonwilliams]

It looks like droppedEntriesCount will only ever have a value (and be shown in options) on the first set of delivered entries. After that its undefined, I guess this is because you're unlikely to have dropped entries once you start emitting them through the observer (they no longer need to be buffered).

This should be mentioned on MDN if so. I've raised an issue here:
https://issues.chromium.org/issues/339767457

[tunetheweb]

As updated in https://issues.chromium.org/issues/339767457#comment2 I think you're correct that this is only set once. And agree it would be good to document in MDN as not the most obvious!