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.
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.
This type has no properties. It is a double-precision floating-point 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 is a standard time which is considered to be the beginning of the current document's lifetime. It's calculated like this:
Window, the time origin is determined as follows:
Documentis the first one loaded in the
Window, the time origin is the time at which the browser context was created.
WorkerGlobalScope(that is, the script is running as a web worker), the time origin is the moment at which the worker was created.
This type has no methods.
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.
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