performance.now()

The returned value represents the time elapsed since the time origin .

Example

const t0 = performance.now();
doSomething();
const t1 = performance.now();
console.log(`Call to doSomething took ${t1 - t0} milliseconds.`);

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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() .
performance.now()=>システムクロック測定のタイムスタンプ値(マイクロ秒精度).