The performance.now()
method returns a DOMHighResTimeStamp
, measured in milliseconds.
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 thanperformance.now()
in the window who spawned that worker. It used to be the same ast0
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.
It's important to keep in mind that to mitigate potential security threats such as Spectre, browsers typically round the returned value by some amount in order to be less predictable. This inherently introduces a degree of inaccuracy by limiting the resolution or precision of the timer. For example, Firefox rounds the returned time to 1 millisecond increments.
The precision of the returned value is subject to change if/when the security concerns are alleviated through other means.
Syntax
t = performance.now();
Example
const t0 = performance.now();
doSomething();
const 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
performance.now();
// 8781416
// 8781815
// 8782206
// ...
// reduced time precision with `privacy.resistFingerprinting` enabled
performance.now();
// 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.
Starting with Firefox 79, high resolution timers can be used if you cross-origin isolate your document using the Cross-Origin-Opener-Policy
and Cross-Origin-Embedder-Policy
headers:
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 documents. COOP 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.
Specifications
Specification | Status | Comment |
High Resolution Time Level 2The definition of 'performance.now()' in that specification. | Recommendation | Stricter definitions of interfaces and types. |
High Resolution TimeThe definition of 'performance.now()' in that specification. | Recommendation | Initial definition |
Browser compatibility
The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Update compatibility data on GitHub
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
now
|
Chrome Full support 24 Full support 24 No support 21 — 24 Prefixed' Implemented with the vendor prefix: webkit |
Edge
Full support 12 |
Firefox Full support 15 Full support 15 Notes' In Firefox 57.0.4 the accuracy was reduced to 20 microseconds. Notes' In Firefox 59 the accuracy was reduced to 2 milliseconds. Notes' In Firefox 60 the accuracy was increased to 1 millisecond. |
IE
Full support 10 |
Opera
Full support 15 |
Safari
Full support 8 |
WebView Android
Full support Yes |
Chrome Android
Full support 25 |
Firefox Android Full support 15 Full support 15 Notes' In Firefox 57.0.4 the accuracy was reduced to 20 microseconds. Notes' In Firefox 59 the accuracy was reduced to 2 milliseconds. Notes' In Firefox 60 the accuracy was increased to 1 millisecond. |
Opera Android
Full support 14 |
Safari iOS
Full support 9 |
Samsung Internet Android
Full support 1.5 |
Legend
- Full support
- Full support
- See implementation notes.'
- See implementation notes.
- Requires a vendor prefix or different name for use.'
- Requires a vendor prefix or different name for use.
See also
- When milliseconds are not enough: performance.now() from HTML5 Rocks.
performance.now() by Mozilla Contributors is licensed under CC-BY-SA 2.5.