Count Browser Window Framerate with JavaScript

fpsCounter.js

let previousTimeStamp = null, frames = 0;

function counter(timestamp) {
    if (previousTimeStamp == null) {
        previousTimeStamp = parseInt(timestamp / 1000);
    }
    frames = frames + 1;
    const sec = parseInt(timestamp / 1000);
    if (sec !== previousTimeStamp) {
        previousTimeStamp = sec;
        console.log(frames);
        frames = 0;
    }

    window.requestAnimationFrame(counter);
}

window.requestAnimationFrame(counter);