How to calculate FPS using requestAnimationFrame

fps.js

// note(): duration to run the test for in µs
const TEST_DURATION_US = 5_000_000;

let start, prev;
let counter = 0;
let ith_second = 0;

function fps(timestamp) {
    // note(): convert to µs
    const uTS = timestamp * 1000;

    // note(): flag to control the animation
    let done = false;

    if (!start) {
        start = uTS;
    }

    const elapsed = uTS - start;

    // note(): this is to avoid duplicate processing
    if (prev !== uTS) {
        // note(): keep counting upto a second
        const nth_second = parseInt(elapsed / 1_000_000);
        if (nth_second === ith_second) {
            counter += 1;
        } else {
            // note(): every second print fps and reset counter
            ith_second = nth_second;
            console.log(`fps: ${counter}`);
            counter = 0;
        }
    }

    // note(): stop after a certain duration
    if (elapsed > TEST_DURATION_US) {
        done = true;
    }

    prev = uTS;
    if (!done) {
        window.requestAnimationFrame(fps);
    } else {
        console.log('DONE');
    }
}

window.requestAnimationFrame(fps);