Performance: now() method

Syntax

now()

Parameters

None.

Return value

Returns a DOMHighResTimeStamp measured in milliseconds.

Description

Performance.now vs. Date.now

Unlike Date.now, the timestamps returned by performance.now() are not limited to one-millisecond resolution. Instead, they represent times as floating-point numbers with up to microsecond precision.

Also, Date.now() may have been impacted by system and user clock adjustments, clock skew, etc. as it is relative to the Unix epoch (1970-01-01T00:00:00Z) and dependent on the system clock. The performance.now() method on the other hand is relative to the timeOrigin property which is a monotonic clock: its current time never decreases and isn't subject to adjustments.

performance.now specification changes

The semantics of the performance.now() method changed between High Resolution Time Level 1 and Level 2.

The performance.now() method used to be relative to performance.timing.navigationStart property from the Navigation Timing specification. This changed 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();

Ticking during sleep

The specification (Level 2) requires that performance.now() should tick when the operating system sleeps or the browser process freezes in any other way. It appears that only browsers on Windows keep ticking during sleep. Relevant browser bugs for other operating systems:

• Chrome/Chromium (bug)
• Firefox (bug)
• Safari/WebKit (bug)

Depending on your use case, this divergence may or may not be significant. For example, if you are timing short operations such as loading an image, during which time the system is unlikely to sleep, this may not cause any issues. If you are timing a long operation, you may find Date.now() more useful to avoid these limitations, since the high precision of performance.now() may not be as critical anyway.

More details can also be found in the specification issue hr-time#115.

Security requirements

To offer protection against timing attacks and fingerprinting, performance.now() is coarsened based on whether or not the document is cross-origin isolated.

• Resolution in isolated contexts: 5 microseconds
• Resolution in non-isolated contexts: 100 microseconds

You can use the Window.crossOriginIsolated and WorkerGlobalScope.crossOriginIsolated properties to check if the document is cross-origin isolated:

if (crossOriginIsolated) {
  // Use measureUserAgentSpecificMemory
}

Examples

Using performance.now()

To determine how much time has elapsed since a particular point in your code, you can do something like this:

const t0 = performance.now();
doSomething();
const t1 = performance.now();
console.log(`Call to doSomething took ${t1 - t0} milliseconds.`);

Specifications

Browser compatibility

See also

• Performance.timeOrigin