performancenow

Performance.now() is a method of the Web Performance API that returns a high-resolution timestamp representing the number of milliseconds elapsed since the start of the navigation to the document. The returned value is a floating-point number with sub-millisecond precision and is monotonic, meaning it won’t go backward if the system clock changes.

The timestamp is relative to the document’s time origin, typically the navigationStart moment. To get an absolute wall-clock time, you can add performance.timeOrigin to performance.now(). Because the value is monotonic and not affected by wall-clock adjustments, it is well suited for measuring elapsed time, durations, and performance benchmarks rather than for presenting a real-time clock.

Usage is straightforward: const t0 = performance.now(); // code to measure const t1 = performance.now(); const duration = t1 - t0; This API is available on the window object in browsers and in workers via self.performance, with the same reference point for a given execution context.

Compatibility and scope: Performance.now() is supported in all major browsers, including Chrome, Firefox, Safari, and Edge. It is accessible in scripts running on a page as well as inside workers, making it versatile for measuring timing across different threads.

Limitations and notes: The value is relative to the page’s time origin and is intended for measuring durations, not for giving an exact clock reading. Precision and overhead can vary by device and browser, and the timestamp does not cross-origin or cross-process boundaries in a single, unified way. For robust measurements, repeated trials and averaging are recommended.