The performance.now()
method returns a DOMHighResTimeStamp
, measured in milliseconds, accurate to five thousandths of a millisecond (5 microseconds).
The returned value represents the time elapsed since the time origin (the PerformanceTiming.navigationStart
property). In a web worker, the time origin is the time that its execution context (e.g. thread or process) is created. In a window, it is the time that the user navigated (or confirmed navigation, if confirmation was needed) to the current document. 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.
Syntax
t = performance.now();
Example
var t0 = performance.now(); doSomething(); 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()
.
Specifications
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 compatibility
Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | 20.0 webkit 24.0 [1] |
(Yes) | 15.0 (15.0) | 10.0 | 15.0 | 8.0 |
on Web workers | 33 | ? | 34.0 (34.0) | ? | ? | ? |
now() in a dedicated worker is now separate from the main context's now() . |
? | ? | 45.0 (45.0) | ? | ? | ? |
Feature | Android | Android Webview | Edge | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile | Chrome for Android |
---|---|---|---|---|---|---|---|---|
Basic support | 4.0 | 25.0 | (Yes) | 15.0 (15.0) | 10.0 | No support | 9 | 25.0 |
on Web workers | ? | (Yes) | ? | 34.0 (34.0) | ? | ? | ? | (Yes) |
now() in a dedicated worker is now separate from the main context's now() . |
? | ? | ? | 45.0 (45.0) | ? | ? | ? | ? |
[1] Windows versions of Chrome 20 through 33 return performance.now()
only to millisecond precision.
See also
- When milliseconds are not enough: performance.now() from HTML5 Rocks.