javascript - 60fps: How to use requestAnimationFrame the right way?

Question

On my website, a related content box should be animated into the viewport when it gets visible.

I’m trying to make my animation as efficient as possible through CSS and JavaScript, so that it doesn’t affects scroll performance negatively.

While the CSS part was simple (using transform, will-change, contain), I’m struggling a bit with when to use window.requestAnimationFrame.

Should I use it only when the class is added to the element or also when the function isScrolledIntoView is called or even inside isScrolledIntoView, when the elements position is measured?

var percentVisible = 0.25;
window.addEventListener('scroll', function(){
relatedContent(related, percentVisible);
}
)

function relatedContent(r, pV){
    window.requestAnimationFrame(function() {
        if(isScrolledIntoView(r, pV)){
            window.requestAnimationFrame(function(){
                r.classList.add("visible");
             }, r)
        }
    }, r)
}

function isScrolledIntoView(el, percentV) {
var elemTop, elemBottom, elemHeight, overhang, isVisible;
/*window.requestAnimationFrame(
function(){*/
elemTop = el.getBoundingClientRect().top;
elemBottom = el.getBoundingClientRect().bottom;
elemHeight = el.getBoundingClientRect().height;
/*}
);*/
overhang = elemHeight * (1 - percentV);

isVisible = (elemTop >= -overhang) && (elemBottom <= window.innerHeight + overhang);
return isVisible;
}

Answer 1

requestAnimationFrame returns a non-zero long that can be used to cancel your request, so instead of writing your own throttle implementation, you can use the following simpler approach to prevent multiple handlers stacking up:

let currentRequest;
document.addEventListener('scroll', function () {
  cancelAnimationFrame(currentRequest);
  currentRequest = requestAnimationFrame(handleScroll);
});