JavaScript Performance Measurement

Written by Swapnil RajaPublished 24 Aug 2026

performance.now gives a far more precise timestamp than Date.now for measuring code.

Date.now() only has millisecond precision and can jump if the clock changes.

performance.now() is designed specifically for timing code.

Example

javascript

const started = performance.now();

let total = 0;
for (let i = 0; i < 10000; i++) {
  total += i;
}

const elapsed = performance.now() - started;

console.log(typeof elapsed);
console.log(elapsed >= 0);

PreviousJavaScript Memoization NextJavaScript Proxy

Example

javascript

function timeIt(fn) {
  const start = performance.now();
  fn();
  return performance.now() - start;
}

const duration = timeIt(function () {
  let total = 0;
  for (let i = 0; i < 50000; i++) {
    total += i;
  }
});

console.log(duration >= 0);
console.log(typeof duration);

Example

javascript

function timeIt(fn) {
  const start = performance.now();
  fn();
  return performance.now() - start;
}

const numbers = Array.from({ length: 1000 }, function (_, i) { return i; });

const loopTime = timeIt(function () {
  let total = 0;
  for (const n of numbers) { total += n; }
});

const reduceTime = timeIt(function () {
  numbers.reduce(function (sum, n) { return sum + n; }, 0);
});

console.log(loopTime >= 0 && reduceTime >= 0);

Example

javascript

function timeIt(fn) {
  const start = performance.now();
  fn();
  return performance.now() - start;
}

const durations = [];
for (let i = 0; i < 3; i++) {
  durations.push(timeIt(function () {
    let total = 0;
    for (let j = 0; j < 10000; j++) { total += j; }
  }));
}

console.log(durations.length);
console.log(durations.every(function (d) { return d >= 0; }));

Complete Example

html

<!DOCTYPE html>
<html>
<head>
  <title>JavaScript Performance Measurement</title>
</head>
<body>

  <h1>Measuring Performance</h1>

  <p id="out"></p>

  <script>
    function timeIt(fn) {
      const start = performance.now();
      fn();
      return performance.now() - start;
    }

    const duration = timeIt(function () {
      let total = 0;
      for (let i = 0; i < 1000000; i++) {
        total += i;
      }
    });

    document.getElementById("out").textContent =
      "That loop took " + duration.toFixed(2) + "ms";
  </script>

</body>
</html>

JavaScript Performance Measurement

Example

Measuring Precisely

Syntax

Timing a Block of Code

Example

Comparing Two Approaches

Example

console.time and console.timeEnd

Example

Run It More Than Once

Example

Measure in the Real Environment

Example

Complete Example

Try It Yourself

Important Points

Conclusion