Floating nav bar that stops fixed to the top of the page

Question

I am trying to mimic something that I have seen a few different times. There is usually a banner with a nav bar below and when the page is scrolled it moves until it touches the

Answer 1

All references to a non-existant navTopPos variable should be changed to reference the topPos variable. Your JS code should now be:

var topPos;
var updatedPos;
window.onscroll = navPos;
if (!topPos) {
    topPos = 120;
}
function navPos() {
    var pos = window.scrollY;
    if (!topPos) {//this line was changed
        topPos = 120;//this line was changed
    }
    var st = document.getElementById('navTest');

    if (st) {
        if (pos < topPos && updatedPos != 'absolute') {
            st.style.position = 'absolute';
            st.style.top = topPos + 'px';//this line was changed
            updatedPos = 'absolute';
            //alert('Switched to absolute');
        } else if (pos >= topPos && updatedPos != 'fixed') {
            st.style.position = 'fixed';
            st.style.top = '0';    
            updatedPos = 'fixed';
            //alert('Switched to fixed');
        }
    }
}
navPos();

The interpreter died somewhere here:

if (!topNavPos) {
    topNavPos = 120;
}

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​ Updated JSFiddle with the necessary changes.