new_performanceobserver_callback.md

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* `callback` {Function}
  * `list` {PerformanceObserverEntryList}
  * `observer` {PerformanceObserver}

`PerformanceObserver` objects provide notifications when new
`PerformanceEntry` instances have been added to the Performance Timeline.

```js
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries());
  observer.disconnect();
});
obs.observe({ entryTypes: ['mark'], buffered: true });

performance.mark('test');
```

Because `PerformanceObserver` instances introduce their own additional
performance overhead, instances should not be left subscribed to notifications
indefinitely. Users should disconnect observers as soon as they are no
longer needed.

The `callback` is invoked when a `PerformanceObserver` is
notified about new `PerformanceEntry` instances. The callback receives a
`PerformanceObserverEntryList` instance and a reference to the
`PerformanceObserver`.