performance.now()

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

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.

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.

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().

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.

• When milliseconds are not enough: performance.now() from HTML5 Rocks.