performance.now()
方法返回
DOMHighResTimeStamp
, measured in milliseconds.
The returned value represents the time elapsed since the time origin .
Bear in mind the following points:
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.
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.
t = performance.now();
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 .
| 规范 | 状态 | 注释 |
|---|---|---|
|
High Resolution Time Level 2
The definition of 'performance.now()' in that specification. |
推荐 | Stricter definitions of interfaces and types. |
|
高分辨率时间
The definition of 'performance.now()' in that specification. |
推荐 | 初始定义 |
| 桌面 | 移动 | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
now
|
Chrome
24
|
Edge 12 |
Firefox
15
|
IE 10 | Opera 15 | Safari 8 | WebView Android Yes | Chrome Android 25 |
Firefox Android
15
|
Opera Android 14 | Safari iOS 9 | Samsung Internet Android 1.5 |
完整支持
见实现注意事项。
要求使用供应商前缀或不同名称。