The Performance API allows high precision measurements that are based on time in potential sub-millisecond resolution and a stable monotonic clock that is not subject to system clock skew or adjustments. The high resolution timers are needed for accurate benchmarking instead of the less precise and non-monotonic
This page provides an overview about how high precision time works within the Performance API and how it compares to
High precision timing is achieved by using the
DOMHighResTimeStamp type for time values. The unit is milliseconds and should be accurate to 5 µs (microseconds). However, if the browser is unable to provide a time value accurate to 5 microseconds, the browser can represent the value as a time in milliseconds accurate to a millisecond. This might occur because of hardware/software constraints, or security and privacy reasons. For more information, see the section on reduced precision below.
All timestamps in the Performance API use the
DOMHighResTimeStamp type. Previously, the Performance API (and other Web APIs) used the
EpochTimeStamp type (previously known as
DOMTimeStamp). These types are now discouraged.
Date.now() as the number of milliseconds elapsed since the epoch, which is defined as the midnight at the beginning of January 1, 1970, UTC. The
performance.now() method on the other hand is relative to the
Performance.timeOrigin property. For more information, see the time origins section below.
Date times are subject to system clock skew or adjustments. This means that the value of time may not always be monotonically increasing. The main purpose of
Date objects is to display time and date information to the user and so many operating systems run a daemon which regularly synchronizes time. It might be that the clock is tweaked a few milliseconds several times per hour.
performance.now() method (and all other
DOMHighResTimeStamp values) provide monotonically increasing time values and aren't subject to clock adjustments. This means that it is guaranteed
DOMHighResTimeStamp values will be at least equal to, if not greater than, the last time you accessed it.
Date.now(); // 1678889977578 performance.now(); // 233936
For measuring performance, calculating precise frame rates (FPS), animation loops, etc., use monotonically increasing high resolution time available with
The Performance API uses the
Performance.timeOrigin property to determine the baseline for performance-related timestamps. All
DOMHighResTimeStamp times are relative to the
In the previous version of the specification (Level 1), the
performance.now() method used to be relative to
performance.timing.navigationStart property from the Navigation Timing specification. However, this changed in a later version of the specification (Level 2) and
performance.now() is now relative to
Performance.timeOrigin which avoids clock change risks when comparing timestamps across webpages.
// Level 1 (clock change risks) currentTime = performance.timing.navigationStart + performance.now(); // Level 2 (no clock change risks) currentTime = performance.timeOrigin + performance.now();
To account for the different time origins in window and worker contexts, you should translate the timestamps coming from worker scripts with the help of the
timeOrigin property, so the timings synchronize for the entire application. See the examples section on the
Performance.timeOrigin page for example code to synchronize time.
To offer protection against timing attacks and fingerprinting,
DOMHighResTimeStamp types are coarsened based on site isolation status.
- Resolution in isolated contexts: 5 microseconds
- Resolution in non-isolated contexts: 100 microseconds
Cross-Origin-Opener-Policy: same-origin Cross-Origin-Embedder-Policy: require-corp
These headers ensure a top-level document does not share a browsing context group with
Cross-Origin-Opener-Policy process-isolates your document and potential attackers
can't access to your global object if they were opening it in a popup, preventing a set
of cross-origin attacks dubbed XS-Leaks.