Difference in values of $(document).height() as compared to $(document).scrollTop() added to $(window).height()

I am currently working on a project where I need to detect when an element enters the view in order to make it fade in. My initial approach was to track the element's vertical position on the page and trigger the fade-in effect when the scroll value was within a certain range.

However, I have encountered a problem with the calculations. The document height always seems to be significantly larger than the combined value of the document's scroll top and window height for each page. This discrepancy varies in magnitude, and I suspect it may be related to a navigation feature that hides the desktop menu and replaces it with a "burger" menu. The extent of this change also varies across pages.

As a result, the expression:

document.body.scrollHeight - $(window).height() == $('body').scrollTop();

always evaluates to false in the console log.

Has anyone else encountered this issue? I am highly confused by this unexpected behavior. I have ensured that all floated elements are cleared, and there are no dynamic elements or nodes being added either by AJAX or any other means.

Any insights or theories on what might be causing this discrepancy would be greatly appreciated. Thank you in advance for your help.

javascriptjqueryhtmlcss

Answer №1

document.height refers to the height of the body tag, while $(window).height() represents the height of the browser window. The calculation on the left side deducts the viewport from the total height, but this does not necessarily indicate the scroll position of the document.

+ - - - - - - +    + - - - - - - +   + - - - - - - +
:             :    :             :   :  Scroll Top :
:             :    :             :   :  4 lines    :
+=============+    :             :   + - - - - - - +
| window      |    :             :  
| height      |    :   document  :  
| 5 lines     |    :   height    :  
+=============+    : - window    :   
:             :    :   height    :
:             :    :   ________  :
: document    :    :   13 lines  :
: height      :    :             :
: (offscreen) :    :             :
: 18 lines    :    + - - - - - - +
:             :
:             :
:             :
+ - - - - - - +

                     13lines != 4lines

For a more accurate starting position of elements below the window, consider using 

$(document).height() - ($(window).scrollTop() + $(window).height());
.

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