I have a div with position \"fixed\" and I want to get the value of its position relative to the whole document while the user scrolls down the page.
So
.offset() gives you the coordinates relative to the whole document.
The .offset() method allows us to retrieve the current position of an element relative to the document. Contrast this with .position(), which retrieves the current position relative to the offset parent. When positioning a new element on top of an existing one for global manipulation (in particular, for implementing drag-and-drop), .offset() is the more useful.
.offset() returns an object containing the properties top and left.
Note: jQuery does not support getting the offset coordinates of hidden elements or accounting for borders, margins, or padding set on the body element.