JavaScript getBoundingClientRect() changes while scrolling

I want to have the exact distance between the Y-coordinate of an element an the Y-value=0, which I consider as the top of the document.

myElement.getBoundingClientRect().top;

But the value of getBoundingClientRect() seems to change while scrolling. How can I get the real distance between myElement and the Y-coordinate=0 (top of document)?

It is because getBoundingClientRect() gets values with respect to the window(only the current visible portion of the page), not the document(whole page).
Hence, it also takes scrolling into account when calculating its values
Basically, document = window + scroll

So, to get the distance between myElement and the Y-coordinate=0 (top of document), you would have add the value of vertical-scroll also:

myElement.getBoundingClientRect().top + window.scrollY;

Source: https://developer.mozilla.org/en-US/docs/Web/API/Element.getBoundingClientRect

getBoundingClientRect needs a bit more care to avoid bugs in scrollY/pageYOffset:

function absolutePosition(el) {
	var
		found,
		left = 0,
		top = 0,
		width = 0,
		height = 0,
		offsetBase = absolutePosition.offsetBase;
	if (!offsetBase && document.body) {
		offsetBase = absolutePosition.offsetBase = document.createElement('div');
		offsetBase.style.cssText = 'position:absolute;left:0;top:0';
		document.body.appendChild(offsetBase);
	}
	if (el && el.ownerDocument === document && 'getBoundingClientRect' in el && offsetBase) {
		var boundingRect = el.getBoundingClientRect();
		var baseRect = offsetBase.getBoundingClientRect();
		found = true;
		left = boundingRect.left - baseRect.left;
		top = boundingRect.top - baseRect.top;
		width = boundingRect.right - boundingRect.left;
		height = boundingRect.bottom - boundingRect.top;
	}
	return {
		found: found,
		left: left,
		top: top,
		width: width,
		height: height,
		right: left + width,
		bottom: top + height
	};
}

The bugs to avoid are:

• scrolling in Android Chrome since Chrome Mobile 43 has wrong values for scrollY/pageYOffset (especially when the keyboard is showing and you scroll).

• Pinch-zoom in Microsoft IE or Edge causes wrong values for scrollY/pageYOffset.

• Some (obsolete) browsers don't have a height/width e.g. IE8

Edit: The above code can be simplified a lot by just using document.body.getBoundingClientRect() instead of adding a div - I haven't tried it though so I am leaving my answer as it stands. Also the body needs margin:0 (reset.css usually does this). This answer simplifies the code down a lot, while still avoiding the bugs in jQuery.offset()!

Edit 2: Chrome 61 introduced window.visualViewport to give correct values for the actual viewport which is probably another way to fix issues; but beware that Android Chrome 66 was still buggy if Settings -> Accessability -> Force enable zoom was ticked (bugs with orientation change, focused inputs, absolutely positioned popup wider than viewport).