Performance: now() method
The performance.now() method returns a high resolution timestamp in milliseconds. It represents the time elapsed since Performance.timeOrigin (the time when navigation has started in window contexts, or the time when the worker is run in Worker and ServiceWorker contexts).
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.
| Changes | Level 1 | Level 2 |
|---|---|---|
| Relative to | performance.timing.navigationStart |
Performance.timeOrigin |
| Triggering conditions | Document fetch or unload prompt (if any). | Creation of the browsing context (if no prior document), unload prompt (if any), or start of the navigation (as defined in HTML, a few steps before fetch). |
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 during sleep. It appears that only Firefox on Windows, and Chromiums on Windows keep ticking during sleep. Relevant browser bugs for other operating systems:
More details can also be found in the specification issue hr-time#115.
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.`);
Security requirements
To offer protection against timing attacks and fingerprinting, performance.now() is coarsened based on site isolation status.
- Resolution in isolated contexts: 5 microseconds
- Resolution in non-isolated contexts: 100 microseconds
Cross-origin isolate your site 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 |
|---|
| High Resolution Time # dom-performance-now |
Browser compatibility
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