Performance APIs

To ensure web applications are fast, it's important to measure and analyze various performance metrics. The Performance API provides important built-in metrics and the ability to add your own measurements to the browser's performance timeline. The performance timeline contains high precision timestamps and can be displayed in developer tools. You can also send its data to analytics end points to record performance metrics over time.

Each performance metric is represented by a single PerformanceEntry. A performance entry has a name, a duration, a startTime, and a type. All performance metrics extend the PerformanceEntry interface and qualify it further.

Most of the performance entries are recorded for you without you having to do anything, and are then accessible either through Performance.getEntries() or (preferably) through PerformanceObserver. For example, PerformanceEventTiming entries are recorded for events that take longer than a set threshold. But the Performance API also enables you to define and record your own custom events, using the PerformanceMark and PerformanceMeasure interfaces.

The main Performance interface is available in both Window and Worker global scopes, and enables you to add custom performance entries, to clear performance entries, and to retrieve performance entries.

The PerformanceObserver interface enables you to listen for various types of performance entry as they are recorded.

For more conceptual information, see the Performance API guides below.

The following interfaces are present in the Performance API:

EventCounts

A read-only map returned by performance.eventCounts containing the number of events which have been dispatched per event type.

LargestContentfulPaint

Measures the render time of the largest image or text block visible within the viewport, recorded from when the page first begins to load.

LayoutShift

Provides insights into the layout stability of web pages based on movements of the elements on the page.

LayoutShiftAttribution

Provides debugging information about elements which have shifted.

NotRestoredReasonDetails

Represents a single reason why a navigated page was blocked from using the back/forward cache (bfcache).

NotRestoredReasons

Provides report data containing reasons why the current document was blocked from using the back/forward cache (bfcache) on navigation.

Performance

Main interface to access performance measurements. Available to window and worker contexts using Window.performance or WorkerGlobalScope.performance.

PerformanceElementTiming

Measures rendering timestamps of specific elements.

PerformanceEntry

An entry on the performance timeline encapsulating a single performance metric. All performance metrics inherit from this interface.

PerformanceEventTiming

Measures latency of events and first input delay (FID).

PerformanceLongAnimationFrameTiming

Provides metrics on long animation frames (LoAFs) that occupy rendering and block other tasks from being executed.

PerformanceLongTaskTiming

Provides metrics on long tasks that occupy rendering and block other tasks from being executed.

PerformanceMark

Custom marker for your own entry on the performance timeline.

PerformanceMeasure

Custom time measurement between two performance entries.

PerformanceNavigationTiming

Measures document navigation events, like how much time it takes to load a document.

PerformanceObserver

Listens for new performance entries as they are recorded in the performance timeline.

PerformanceObserverEntryList

List of entries that were observed in a performance observer.

PerformancePaintTiming

Measures render operations during web page construction.

PerformanceResourceTiming

Measures network loading metrics such as redirect start and end times, fetch start, DNS lookup start and end times, response start and end times for resources such as images, scripts, fetch calls, etc.

PerformanceScriptTiming

Provides metrics on individual scripts causing long animation frames (LoAFs).

PerformanceServerTiming

Surfaces server metrics that are sent with the response in the Server-Timing HTTP header.

TaskAttributionTiming

Identifies the type of task and the container that is responsible for the long task.

VisibilityStateEntry

Measures the timing of page visibility state changes, i.e., when a tab changes from the foreground to the background or vice versa.

The following guides help you to understand key concepts of the Performance API and provide an overview about its abilities:

• Performance data: Collecting, accessing, and working with performance data.
• High precision timing: Measuring with high precision time and monotonic clocks.
• Resource timing: Measuring network timing for fetched resources, such as images, CSS, and JavaScript.
• Navigation timing: Measuring navigation timing of a document.
• User timing: Measuring and recording performance data custom to your application.
• Server timing: Collecting server-side metrics.
• Long animation frame timing: Collecting metrics on long animation frames (LoAFs) and their causes.
• Monitoring bfcache blocking reasons: Reporting on why the current document was blocked from using the back/forward cache (bfcache).

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