Detecting offset of an element on scroll in javasc

Question

I am looking looking to calculate the distance between an element and the top of the document window. On scroll I am able to get the initial value but it does not change. How can I find this value and what the number has changed to on scroll?

JS:

$(function() {
    $(window).scroll(function() {
        var h1 = $("h1");
        console.log(h1.offset().top)
    });
});

HTML:

<div id="cover">   
    <h1>hello sir</h1> 
</div>

Answer 1

If you hate arithmetic (and extra function calls), this should do the trick:

$(function() {
    var h1 = document.getElementsByTagName("h1")[0];
    $(window).scroll(function() {
        console.log(h1.getBoundingClientRect().top);
    });
});

This is exactly what getBoundingClientRect() was made for.

Answer 2

Compare the offset of the <h1> element to how far down the page the user has scrolled. The $(window).scrollTop() function will get you the amount the user has scrolled down so:

$(window).scroll(function() {
  var $h1 = $("h1");
  var window_offset = $h1.offset().top - $(window).scrollTop();
});

Answer 3

You can use this function to get the scroll offset of the window:

function getScrollXY() {
  var scrOfX = 0, scrOfY = 0;
  if( typeof( window.pageYOffset ) == 'number' ) {
    //Netscape compliant
    scrOfY = window.pageYOffset;
    scrOfX = window.pageXOffset;
  } else if( document.body && ( document.body.scrollLeft || document.body.scrollTop ) ) {
    //DOM compliant
    scrOfY = document.body.scrollTop;
    scrOfX = document.body.scrollLeft;
  } else if( document.documentElement && ( document.documentElement.scrollLeft || document.documentElement.scrollTop ) ) {
    //IE6 standards compliant mode
    scrOfY = document.documentElement.scrollTop;
    scrOfX = document.documentElement.scrollLeft;
  }
  return [ scrOfX, scrOfY ];
}

Then you can use the offsets in your function to determine the real position of your element:

$(function() {
    $(window).scroll(function() {
        var h1 = $("h1");
        var offs = getScrollXY();
        console.log(h1.offset().top - offs[1]);
    });
});

Theoretically, this should work in all browsers, but, frankly, I didn't do too much testing.