PerformanceObserver.observe()

The observe() method of the PerformanceObserver interface is used to specify the set of performance entry types to observe. The performance entry types are specified as an array of DOMString objects, each naming one entry type; the type names are documented in Performance entry type names in PerformanceEntry.entryType.

When a matching performance entry is recorded, the performance observer's callback function—set when creating the PerformanceObserver—is invoked.

options

A PerformanceObserverInit dictionary with the following possible members:

• entryTypes: An array of DOMString objects, each specifying one performance entry type to observe.  May not be used together with the "type" or "buffered" options.
• type: A single DOMString specifying exactly one performance entry type to observe.  May not be used together with the entryTypes option.
• buffered: A Boolean flag to indicate whether buffered entries should be queued into the observer's buffer.  Must be used only with the "type" option.

See PerformanceEntry.entryType for a list of valid performance entry type names. Unrecognized types are ignored, though the browser may output a warning message to the console to help developers debug their code. If no valid types are found, observe() has no effect.

This example creates and configures two PerformanceObservers; one watches for "mark" and "frame" events, and the other watches for "measure" events.

var observer = new PerformanceObserver(function(list, obj) {
  var entries = list.getEntries();
  for (var i=0; i < entries.length; i++) {
    // Process "mark" and "frame" events
  }
});
observer.observe({entryTypes: ["mark", "frame"]});

function perf_observer(list, observer) {
  // Process the "measure" event
}
var observer2 = new PerformanceObserver(perf_observer);
observer2.observe({entryTypes: ["measure"]});