javascript - Add class when element reaches top of viewport

Question

I am trying to add a class to the header when an element reaches the top of the viewport but I cannot seem to find out why it is not working. I have no errors and I have checked to see that jquery is fetching the offsets and it is. Any help would be great. I would also like to know how to extend this code to any number of section's rather than stating just 6.

JS FIDDLE

$(document).ready(function () {
    var project1 = $('section:nth-child(1)').offset();
    var project2 = $('section:nth-child(2)').offset();
    var project3 = $('section:nth-child(3)').offset();
    var project4 = $('section:nth-child(4)').offset();
    var project5 = $('section:nth-child(5)').offset();
    var project6 = $('section:nth-child(6)').offset();
    var $window = $(window);

    $window.scroll(function () {
        if ($window.scrollTop() >= project1) {
            $("header").removeClass().addClass("project1");
        }
        if ($window.scrollTop() >= project2) {
            $("header").removeClass().addClass("project2");
        }
        if ($window.scrollTop() >= project3) {
            $("header").removeClass().addClass("project3");
        }
        if ($window.scrollTop() >= project4) {
            $("header").removeClass().addClass("project4");
        }
        if ($window.scrollTop() >= project5) {
            $("header").removeClass().addClass("project5");
        }
        if ($window.scrollTop() >= project6) {
            $("header").removeClass().addClass("project6");
        }
    });
});

Answer 1

The method .offset(), returns an object containing the properties top and left:

{top: 1808, left: 8}

Therefore you need to access the top property in your conditional statements.

Change

if ($window.scrollTop() >= project1) { ... }

to:

if ($window.scrollTop() >= project1.top) { ... }

Updated Example

---

As a side note, $('section:nth-child(1)').offset() will be undefined because the section element isn't the first element (the <header> is). Use :nth-of-type rather than :nth-child. Since you're using jQuery, eq() would work too.

$(document).ready(function() {
    var project1 = $('section:nth-of-type(1)').offset();
    var project2 = $('section:nth-of-type(2)').offset();
    var project3 = $('section:nth-of-type(3)').offset();
    var project4 = $('section:nth-of-type(4)').offset();
    var project5 = $('section:nth-of-type(5)').offset();
    var project6 = $('section:nth-of-type(6)').offset();
    var $window = $(window);
    
    $window.scroll(function() {
        if ( $window.scrollTop() >= project1.top) {
            $("header").removeClass().addClass("project1");
        }
        if ( $window.scrollTop() >= project2.top ) {
            $("header").removeClass().addClass("project2");
        }
        if ( $window.scrollTop() >= project3.top ) {
            $("header").removeClass().addClass("project3");
        }
        if ( $window.scrollTop() >= project4.top ) {
            $("header").removeClass().addClass("project4");
        }
        if ( $window.scrollTop() >= project5.top ) {
            $("header").removeClass().addClass("project5");
        }
        if ( $window.scrollTop() >= project6.top ) {
            $("header").removeClass().addClass("project6");
        }
    });
});

header {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: 100px;
    background: #000;
}
header.project1 {
    background: red;
}
header.project2 {
    background: orange;
}
header.project3 {
    background: blue;
}
header.project4 {
    background: green;
}
header.project5 {
    background: red;
}
header.project6 {
    background: blue;
}
section {
    height: 900px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<header></header>
<section>Section 1</section>
<section>Section 2</section>
<section>Section 3</section>
<section>Section 4</section>
<section>Section 5</section>
<section>Section 6</section>