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Detecting by how much user has scrolled

I have an image pop-up ability on my website, in order to show users the full resolution picture when they click on a smaller version on the page.
This is the current CSS that positions it:
div#enlargedImgWrapper {
position: absolute;
top: 30px;
left: 55px;
z-index: 999;
}
The problem now is that if I click on an image further down the page, the window still appears in the top left corner of the page, where I can't see it until I scroll back up. I need it to appear relative to the window, whatever its current position relative to the document is.
Note: I don't want to use position: fixed; as some images might be taller than the screen, so I want users to be able to scroll along the image as well.
My idea was to use JS to change the top value:
var scrollValue = ???;
document.getElementById('enlargedImgWrapper').style.top = scrollValue+30 + 'px';
How can I detect by how much the user has scrolled down the page (var scrollValue)?
Or is there a 'better' way to do this?
Edit: if possible I would like to do this without jQuery.

Pure JavaScript uses scrollTop and scrollLeft:
var scrollLeft = (window.pageXOffset !== undefined) ? window.pageXOffset : (document.documentElement || document.body.parentNode || document.body).scrollLeft;
var scrollTop = (window.pageYOffset !== undefined) ? window.pageYOffset : (document.documentElement || document.body.parentNode || document.body).scrollTop;
https://developer.mozilla.org/en-US/docs/Web/API/Element.scrollTop
jQuery version:
var scrollLeft = $(window).scrollLeft() ;
var scrollTop = $(window).scrollTop() ;
What you need is this:
document.getElementById('enlargedImgWrapper').style.top = (scrollTop+30) + 'px';

document.getElementById('enlargedImgWrapper').scrollTop;
MDN
This property's value equals the current vertical offset of the
content within the scrollable range. Although you can set this
property to any value, if you assign a value less than 0, the property
is set to 0. If you assign a value greater than the maximum value, the
property is set to the maximum value.
You can set this property inline, but the results might be
inconsistent while the document is loading.
scrollTop property

Javascript: check IF page is at the top

Is there a way to check, with JavaScript, if the page is at scroll(0,0)?
Reason being I've got a full page slider that I need to pause the second the page is not at origin.
And it might not necessarily be because the page is being scrolled live as I've got internal HTML # links that would load the page right to a scrolled point without actually scrolling.
So the check needs to be is the page not at the top, as opposed to, has the page been scrolled.

Try this:
document.body.scrollTop === 0

You can check if window.scrollY (the number of pixels the window has scrolled vertically) is equal to 0. If you want to check if the window has been scrolled to its leftermost, you can check if window.scrollX (the number of pixels the window has scrolled horizontally) is equal to 0. If you combine the two, it will ensure the window is at (0,0) position.
if(window.scrollY==0){
//user scrolled to the top of the page
}
if(window.scrollX==0){
//user scrolled to the leftmost part of the page
}
if(window.scrollY==0&&window.scrollX==0){
//user scrolled to the leftmost part of the top of the page—i.e., they are at position (0, 0)
}
Demo:
var goToTop = document.querySelector('#backToTop');
goToTop.addEventListener("click", function(e){
window.scroll({top: 0, left: 0, behavior: 'smooth'});
//scroll smoothly back to the top of the page
});
window.addEventListener("scroll", function(){
if(window.scrollY==0){
//user is at the top of the page; no need to show the back to top button
goToTop.style.display = "";
} else {
goToTop.style.display = "block";
}
});
body,html{height:3000px;position:relative;margin:0}#footer{position:absolute;width:100%;bottom:0;left:0;right:0;background-color:#1e90ff;text-align:center}#backToTop{position:fixed;right:0;bottom:0;display:none;z-index:1}#header{position:absolute;width:100%;top:0;left:0;right:0;background-color:#1e90ff;text-align:center}
<div id="header">Header</div>
<button id="backToTop">Back To Top</button>
<div id="footer">Footer</div>
For better browser compatibility, use window.pageYOffset instead of window.scrollY and window.pageXOffset instead of window.scrollX.
The following code can be used in cases when full browser compatability is necessary (i.e., IE < 9):
var x = (window.pageXOffset !== undefined)
? window.pageXOffset
: (document.documentElement || document.body.parentNode || document.body).scrollLeft;
//number of pixels scrolled horizontally (work with this value instead of window.scrollX or window.pageXOffset)
var y = (window.pageYOffset !== undefined)
? window.pageYOffset
: (document.documentElement || document.body.parentNode || document.body).scrollTop;
//number of pixels scrolled vertically (work with this value instead of window.scrollY or window.pageYOffset)

Updated Answer for 2019
document.body.scrollTop is deprecated and doesn't work at all in Chrome anymore. The best way to solve this is simply looking at all three possibilities to have a cross browser solution.
!window.pageYOffset
One of these three should work on all browser types. If the value equals 0, your at the top of the viewport.

i think you can get the position using jQuery $(window).scrollTop()

How do I find the current window offset using javascript?

When I open a page, the window offset would be 0 but when i scroll through the page, the offset of the window would increase correspondingly? How can I find the window's offset at any particular point of my web page?

You can get the scroll offset of the window by using window.scrollX & window.scrollY. See the MDN article on scrollY for more information.
window.scrollY; // the current vertical scroll offset of the window
Just thought I would provide the non-jQuery version.

I don't know if I get the question, but for getting scroll offset use:
var scrollOffset = $(window).scrollTop();

The .offset() method is undefined for window object or window element therefore you
should use the .scrollTop() method to solve your issue :)

Get distance of window from the top of document on page load (javascript only)

I've been having trouble calculating this on page load. It should only take one line but I can't seem to get it.

could you elaborate on what the "distance of window to document" means? if you are looking for screen height/width:
window.screen.height
window.screen.width
or
window.screen.availHeight
window.screen.availWidth
for vertical scroll position use:
window.pageYOffset
window.scrollY
I'm pretty sure they are equivalent i.e.
window.pageYOffset == window.scrollY; // always true

DSOC (document scroll offset coordinates) can be found with window.pageXOffset and window.pageYOffset. In your case, you want window.pageYOffset.
More details here:
http://www.javascriptkit.com/javatutors/static2.shtml

I realize that this is an old question, but I had the same problem and sought out a way to fix it. It appears that window.scrollY is not set directly on page load. However, if you do the following, it will register the correct value:
window.setTimeout(function() { console.log(window.scrollY); }, 10);
I've determined that the extra 10ms allows for the document to load and for it to set the window.scrollY value.

How to find (in javascript) the current "scroll" offset in mobile safari / iphone

I'd like to know the x/y offset of the how far the user has "scrolled" within the viewport in mobile safari on the iphone.
Put another way, if I (through javascript) reloaded the current page, I'd like to find the values I'd need to pass into window.scrollTo(...) in order to reposition the document/viewport as it is currently.
window.pageXOffset always reports 0
jquery's $('body').scrollTop() always reports 0
events have a pageX, but this won't account for the scrolling of the page that happens after you release your finger if your gesture was to "flick" the page up/down. Namely, it'll give me a point when the finger leaves the screen, but that doesn't always match where the page will be after it's finished scrolling.
Any pointers?

window.pageYOffset should give you the current scroll offset. There's window.pageXOffset if you need it too.

I had the same problem... this did the trick:
var scrollX = window.pageXOffset; var scrollY = window.pageYOffset;
Got it from this question: Get current position of the viewport in Mobile (iPhone) Safari

This will indeed work:
var scrollX = window.pageXOffset;
var scrollY = window.pageYOffset;
If you are viewing content in an iFrame (which is common in WebViews for instance), then you will need to add parent:
var scrollX = parent.window.pageXOffset;
Also note that these values are not writeable. So to change the scroll position, you will need to use window.scrollTo method:
var scrollX = window.pageXOffset;
var scrollY = window.pageYOffset;
window.scrollTo(scrollX -100, scrollY -100);

Have you tried the pure js way?
document.body.scrollTop

Here is a simple code to find if the device is iphone and also to change the scroll position to specific position based on some action. I had an issue with iphone only when I click and open an image as a popup, because vertical scroll goes down than where I wanted to be. So I wrote this code and it solved the issue.
if((navigator.userAgent.match(/iPhone|iPad|iPod/i))){
('#tutorial-landlord').click(function(){
window.scroll(100, 1500); // x, y (horizontal, vertical position)
});
}
To find the scroll position. Just go to console in chrome and write
window.scrollY for vertical and see how the position changes so note that number and give it in place of x and y

I had the same issue on iPad. Just desactivate the console. The viewport height changes when the console is open in Safari.

If for whatever reason pageXOffset and pageYOffset fail, the solution is straightforward, but rather silly:
// force an element to top left of the page
var topLeftMarker = document.createElement("span");
topLeftMarker.style.position = "absolute";
topLeftMarker.style.left = "0";
topLeftMarker.style.top = "0";
document.body.appendChild(topLeftMarker)
function scrollOffset() {
// getBoundingClientRect() returns the rectangle of the element in viewport space,
// which *is* scrollLeft and scrollTop
const rect = topLeftMarker.getBoundingClientRect();
return { x: rect.left, y: rect.top }
}