DOMHighResTimeStamp - Web APIs 编辑

The DOMHighResTimeStamp type is a double and is used to store a time value in milliseconds. This type can be used to describe a discrete point in time or a time interval (the difference in time between two discrete points in time).

The time, given in milliseconds, should be accurate to 5 µs (microseconds), with the fractional part of the number indicating fractions of a millisecond. However, if the browser is unable to provide a time value accurate to 5 µs (due, for example, to hardware or software constraints), the browser can represent the value as a time in milliseconds accurate to a millisecond. Also note the section below on reduced time precision controlled by browser preferences to avoid timing attacks and fingerprinting.

Further, if the device or operating system the user agent is running on doesn't have a clock accurate to the microsecond level, they may only be accurate to the millisecond.

Reduced time precision

To offer protection against timing attacks and fingerprinting, the precision of time stamps might get rounded depending on browser settings. In Firefox, the privacy.reduceTimerPrecision preference is enabled by default and defaults to 20 µs in Firefox 59; in 60 it will be 2ms.

// reduced time precision (2ms) in Firefox 60
event.timeStamp
// 1519211809934
// 1519211810362
// 1519211811670
// ...

// reduced time precision with `privacy.resistFingerprinting` enabled
event.timeStamp;
// 1519129853500
// 1519129858900
// 1519129864400
// ...

In Firefox, you can also enable privacy.resistFingerprinting, the precision will be 100ms or the value of privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.

Properties

This type has no properties. It is a double-precision floating-point value.

Value

The value of a DOMHighResTimeStamp is a double-precision floating-point number which describes the number of milliseconds (accurate to within 5 microseconds if the device supports it) elapsed between two points in time. The starting time can be either a specific time determined by the script for a site or app, or the time origin.

The time origin

The time origin is a standard time which is considered to be the beginning of the current document's lifetime. It's calculated like this:

• If the script's global object is a Window, the time origin is determined as follows:
  • If the current Document is the first one loaded in the Window, the time origin is the time at which the browser context was created.
  • If during the process of unloading the previous document which was loaded in the window, a confirmation dialog was displayed to let the user confirm whether or not to leave the previous page, the time origin is the time at which the user confirmed that navigating to the new page was acceptable.
  • If neither of the above determines the time origin, then the time origin is the time at which the navigation responsible for creating the window's current Document took place.
• If the script's global object is a WorkerGlobalScope (that is, the script is running as a web worker), the time origin is the moment at which the worker was created.
• In all other cases, the time origin is undefined.

Methods

This type has no methods.

Usage notes

You can get the current timestamp value—the time that has elapsed since the context was created—by calling the performance method now(). This method is available in both Window and Worker contexts.

Example

To determine how much time has elapsed since a particular point in your code, you can do something like this:

let startTime = performance.now();

/* ... do things for a while ... */

let elapsedTime = performance.now() - startTime;

Upon completion, the value of elapsedTime is the number of milliseconds that have elapsed since you recorded the starting time in line 1.

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Navigation Timing API
• Performance
• performance.now()