performance.now()

Introduction

performance.now() returns a high-resolution timestamp (in milliseconds) with sub-millisecond precision — far more accurate than Date.now().

Basic Usage

const start = performance.now();

// Code to measure
for (let i = 0; i < 1000000; i++) {
  Math.sqrt(i);
}

const end = performance.now();
console.log(`Took ${end - start}ms`);

Date.now() vs performance.now()

Feature	Date.now()	performance.now()
Precision	Milliseconds	Microseconds (fractional ms)
Monotonic	No (can jump with system clock)	Yes (always increases)
Purpose	Calendar time	Code performance measurement

// Date.now() can be affected by clock changes
const t1 = Date.now();
await someAsyncOperation();
const t2 = Date.now(); // Could be wrong if clock adjusted!

// performance.now() is monotonic — always reliable
const p1 = performance.now();
await someAsyncOperation();
const p2 = performance.now(); // Always accurate

Measuring Async Operations

async function measureAsync() {
  const start = performance.now();

  await fetch('/api/data');
  await processData();

  const duration = performance.now() - start;
  console.log(`Total async operation: ${duration.toFixed(2)}ms`);
}

Measuring Individual Operations

function measureOperation(fn, iterations = 1000) {
  const start = performance.now();

  for (let i = 0; i < iterations; i++) {
    fn();
  }

  const total = performance.now() - start;
  const average = total / iterations;
  console.log(`Total: ${total.toFixed(2)}ms, Average: ${average.toFixed(4)}ms`);
}

// Usage
measureOperation(() => Math.sqrt(42));
measureOperation(() => [1, 2, 3].map(n => n * 2), 10000);

Summary

Sub-millisecond precision (fractional milliseconds)
Monotonic clock — not affected by system time changes
Ideal for measuring code performance
Use with Date.now() for timestamps; use performance.now() for durations

performance.now()

performance.now()

Introduction

Basic Usage

Date.now() vs performance.now()

Measuring Async Operations

Measuring Individual Operations

Summary

Further Reading