The performance.now() method returns a DOMHighResTimeStamp, measured in milliseconds.

The timestamp is not actually high-resolution. To mitigate security threats such as Spectre, browsers currently round the results to varying degrees. (Firefox started rounding to 1 millisecond in Firefox 60.) Some browsers may also slightly randomize the timestamp. The precision may improve again in future releases; browser developers are still investigating these timing attacks and how best to mitigate them.

Note: This feature is available in Web Workers.

The returned value represents the time elapsed since the time origin.

Bear in mind the following points:

  • In dedicated workers created from a Window context, the value in the worker will be lower than performance.now() in the window who spawned that worker. It used to be the same as t0 of the main context, but this was changed.
  • In shared or service workers, the value in the worker might be higher than that of the main context because that window can be created after those workers.


t = performance.now();


var t0 = performance.now();
var t1 = performance.now();
console.log("Call to doSomething took " + (t1 - t0) + " milliseconds.");

Unlike other timing data available to JavaScript (for example 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 unlike Date.now(), the values returned by Performance.now() always increase at a constant rate, independent of the system clock (which might be adjusted manually or skewed by software like NTP). Otherwise, performance.timing.navigationStart + performance.now() will be approximately equal to Date.now().

Reduced time precision

To offer protection against timing attacks and fingerprinting, the precision of performance.now() might get rounded depending on browser settings.
In Firefox, the privacy.reduceTimerPrecision preference is enabled by default and defaults to 1ms.

// reduced time precision (1ms) in Firefox 60
// 8781416
// 8781815
// 8782206
// ...

// reduced time precision with `privacy.resistFingerprinting` enabled
// 8865400
// 8866200
// 8866700
// ...

In Firefox, you can also enable privacy.resistFingerprinting — this changes the precision to 100ms or the value of privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.


Specification Status Comment
High Resolution Time Level 2
The definition of 'performance.now()' in that specification.
Candidate Recommendation Stricter definitions of interfaces and types.
High Resolution Time
The definition of 'performance.now()' in that specification.
Recommendation Initial definition

Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
Basic support 24 Yes 15
In Firefox 57.0.4 the accuracy was reduced to 20 microseconds.
In Firefox 59 the accuracy was reduced to 2 milliseconds.
10 15 8
Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android iOS Safari Samsung Internet
Basic support Yes 25 Yes 15
In Firefox 57.0.4 the accuracy was reduced to 20 microseconds.
In Firefox 59 the accuracy was reduced to 2 milliseconds.
No 9 ?

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.