jQuery - Change background colour on scroll

Question

I want to transition the background colour of a fixed header element on scroll. So as a user scrolls down a full page block website, the header subtly changes to complement the block colours. I have almost achieved this on a Pen, however I can't quite work out how to measure how much has been scrolled as a flag for when to change.

Some extra info: The scroll amount to change at is 400px. The background colours are stored and fetched in an array. For reference my jQuery code is below:

$(document).ready(function(){
  var bgArray = ["#252525","#333333","#454545","#777777"];  
  var scrollHeight = 400;
  var scrolled = $(window).scrollTop(); //What is this measuring?

  $(window).scroll(function() { //Can these conditions be neatened into one function?
    if(scrolled < scrollHeight) {
      $('header').css('background', bgArray[0]);
    }
    if(scrolled > scrollHeight) { // i.e more than 400px
      $('header').css('background', bgArray[1]);
    }
    // and so on (800, 1200...)
  })
})

Please refer to the Pen for full code. Any suggestions are greatly appreciated!

Answer 1

Updated Solution (2019)

To set a background for the header based on the current block in view below the header while scrolling:

• because header has fixed position, we can get the amount by which window has scrolled by using $header.offset().top,

• (index of the current block in view) is the ratio of (the amount by which window has scrolled) to the (height of each block),

• now adjusting for the height of the header, the index of the current block in view is Math.floor(($header.offset().top + headerHeight) / sectionHeight).

See simplified demo below:

$(function() {
  var $header = $('header'),
    $window = $(window),
    bgArray = ["#252525", "red", "blue", "green"],
    headerHeight = 50,
    sectionHeight = 400;

  $window.scroll(function() {
    $header.css('background', bgArray[Math.floor(($header.offset().top + headerHeight)
        / sectionHeight)]);
  });
});

:root {
  --header: 50px; /* header height */
  --block: 400px; /* block height */
}

* {
  box-sizing: border-box; /* include padding in width / height calculations */
}

body {
  margin: 0; /* reset default margin of body */
}

header {
  height: var(--header); /* sets height of header */
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  color: #FFF;
  padding: 12px 0;
  background: #252525; /* initial background */
  transition: background 1s ease;
}

.container {
  margin: 0 auto;
}

.wrap>div {
  height: var(--block); /* sets height of each block */
  text-align: center;
}

p {
  margin: 0; /* reset margin of p */
}

.block-1 {
  background: #27AACC;
  color: #FFF;
}

.block-2 {
  background: #668E99;
  color: #FFF;
}

.block-3 {
  background: #4AFFC1;
  color: #444;
}

.block-4 {
  background: #FF8F8A;
  color: #FFF;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<header>
  <div class="container">
    Website Title.
  </div>
</header>

<div class="wrap">
  <div class="block-1">
    <div class="container">
      <p>This pen was made to solve a problem on a project...</p>
    </div>
  </div>
  <div class="block-2">
    <div class="container">
      <p>...I needed a sticky header with thr right bg colour.</p>
    </div>
  </div>
  <div class="block-3">
    <div class="container">
      <p>But this conflicted with the footer, which was the same colour...</p>
    </div>
  </div>
  <div class="block-4">
    <div class="container">
      <p>So the solution was to subtley change the header's bg on scroll</p>
    </div>
  </div>
</div>

---

Original Solution

Check the top of each block with respect to how much the window has been scrolled (scrollTop) using $(window).scrollTop() > $('.block-1').offset().top. So now we can use this to change color on entering the block - see demo below:

$(document).ready(function() {
  var $header = $('header'),
    $window = $(window),
    bgArray = ["#252525", "#333333", "#454545", "#777777"],
    headerHeight = $header.outerHeight();

  $window.scroll(function() {
    for (var i = 1; i < 5; i++) {
      if ($window.scrollTop() + headerHeight > $('.block-' + i).offset().top) {
        $header.css('background', bgArray[i - 1]);
      }
    }
  });
});

@import url('https://fonts.googleapis.com/css?family=Roboto:300,400,700');
body {
  font-family: 'Roboto', sans-serif;
  font-size: 30px;
  font-weight: 300;
  margin-top: 0;
}

header {
  width: 100%;
  height: 50px;
  line-height: 50px;
  position: fixed;
  font-size: 24px;
  font-weight: 700;
  color: #FFF;
  padding: 12px 0;
  margin: 0;
  background: #252525;
  transition: background 1s ease;
}

.wrap {
  padding-top: 74px;
  margin: 0;
}

.container {
  width: 960px;
  margin: 0 auto;
  overflow: hidden;
}

.block-1,
.block-2,
.block-3,
.block-4 {
  height: 400px;
  text-align: center;
}

p {
  margin-top: 185px;
}

.block-1 {
  background: #27AACC;
  color: #FFF;
}

.block-2 {
  background: #668E99;
  color: #FFF;
}

.block-3 {
  background: #4AFFC1;
  color: #444;
}

.block-4 {
  background: #FF8F8A;
  color: #FFF;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<header>
  <div class="container">
    Website Title.
  </div>
</header>
<div class="wrap">
  <div class="block-1">
    <div class="container">
      <p>This pen was made to solve a problem on a project...</p>
    </div>
  </div>
  <div class="block-2">
    <div class="container">
      <p>...I needed a sticky header with thr right bg colour.</p>
    </div>
  </div>
  <div class="block-3">
    <div class="container">
      <p>But this conflicted with the footer, which was the same colour...</p>
    </div>
  </div>
  <div class="block-4">
    <div class="container">
      <p>So the solution was to subtley change the header's bg on scroll</p>
    </div>
  </div>
</div>

Note that this solution needlessly loops through the sections on each scroll update called by the browser - and I don't like the look of it.

Answer 2

you are using scrolled as a fixed variable you should use it directly in your condition

this will make it dynamic for all elements inside wrap div

$(document).ready(function(){
var bgArray = ["#252525","#333333","#454545","#777777"];
$(window).scroll(function() { 
    for(var i = 1; i < bgArray.length; i++) {
      if ($(window).scrollTop() > $('.wrap div:nth-child(' + i + ')').offset().top) {
        $('header').css('background', bgArray[i-1]);        
      }
    }
  });
})

Answer 3

Try Like this:

$(document).ready(function(){
  var bgArray = ["#252525","#333333","#454545","#777777"];  
  var scrollHeight = 400;

  $(window).scroll(function() {

    var scrolled = $(window).scrollTop(); 

    var index=Number((scrolled/scrollHeight).toFixed());

    if(bgArray[index]!=undefined)
        $('header').css('background', bgArray[index]);

  });
})

This is current scroll, so it should be inside: $(window).scrollTop()