I'm doing a progress bar depending on how much you scroll. Which of these two should I use?

function getScrollValue() {
    return ((parseInt(document.body.getBoundingClientRect().top) * -1));
}

or

function getScrollValue() {
    return window.pageYOffset;
}

I'm doing a progress bar depending on how much you scroll. Which of these two should I use?

function getScrollValue() {
    return ((parseInt(document.body.getBoundingClientRect().top) * -1));
}

or

function getScrollValue() {
    return window.pageYOffset;
}

• javascript

• IntersectionObserver is probably the better solution for that. – str Commented Dec 27, 2018 at 14:56
• 1 Use window.pageYOffset (IntersectionObserver is not supported by IE) – Jakob E Commented Dec 27, 2018 at 14:59
• @JakobE There is a polyfill. – str Commented Dec 27, 2018 at 15:01
• 1 Yep - but in this case (just monitoring page scroll) it's overkill (the polyfill also uses scroll listeners). I would use scroll listeners and passive events if available – Jakob E Commented Dec 27, 2018 at 15:09
• @JakobE IntersectionObserver exists because the naive solution to scroll handlers often impose performance problems. It is not overkill to enhance performance for capable browsers while providing a polyfill for heavily outdated browsers. – str Commented Dec 27, 2018 at 15:12

2 Answers 2

Besides of window.pageYOffset being the more specialized API as well as a more performant solution for detecting the scrolling value, using getBoundingClientRect().top for attempting the same thing can have unexpected results if, for instance, the body has some margin-top that offsets its position.

EDIT

I've included the following jsPerf Test that shows a substantial difference of performance between the two alternatives, as well as how a third (older) API fares against them.

A simple example using the progress bar tag

/* test if passive events are supported */
var supportsPassive = false;
try { var opts = Object.defineProperty({}, 'passive', { get: function() { supportsPassive = true; }});
  window.addEventListener("testPassive", null, opts);
  window.removeEventListener("testPassive", null, opts);
} catch (e) {}

/* progress element */
const progress = document.querySelector('progress');

/* scroll listener */
addEventListener('scroll', e => {
    progress.value = pageYOffset/(document.body.clientHeight - innerHeight);
}, supportsPassive ? { passive: true } : false);

/* stretch and fix progress */ 
progress { width: 100%; position: fixed; }

/* add some scroll to the page */
body { margin: 0; height: 20000px; }

<progress value="0"></progress>