Changing color when specific div is visible [closed]

Answer 1

I had to tweak the code a little so the code could be more D.R.Y. The classes are now replaced by color class.

$.fn.isInViewport = function() {
  var elementTop = $(this).offset().top;
  var elementBottom = elementTop + $(this).outerHeight();

  var viewportTop = $(window).scrollTop();
  var viewportBottom = viewportTop + $(window).height();

  return elementBottom > viewportTop && elementTop < viewportBottom;
};

$(window).on('resize scroll', function() {
  $('.color').each(function() {
      var activeColor = $(this).attr('id');
    if ($(this).isInViewport()) {
      $('#fixed-' + activeColor).addClass(activeColor + '-active');
    } else {
      $('#fixed-' + activeColor).removeClass(activeColor + '-active');
    }
  });
});

html,
body {
  height: 100%;
}

.fixed-red,
.fixed-green,
.fixed-blue {
  width: 30px;
  height: 30px;
  position: fixed;
  top: 10px;
  left: 10px;
  background: #333;
}

.fixed-green {
  top: 50px;
}

.fixed-blue {
  top: 90px;
}

.red-active {
  background: #f00;
}

.green-active {
  background: #0f0;
}

.blue-active {
  background: #00f;
}

.color {
  width: 100%;
  height: 100%;
}

#red {
  background: #900;
}

#green {
  background: #090;
}

#blue {
  background: #009;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="fixed-red" class="fixed-red red-active"></div>
<div id="fixed-green" class="fixed-green"></div>
<div id="fixed-blue" class="fixed-blue"></div>
<div id="red" class="color"></div>
<div id="green" class="color"></div>
<div id="blue" class="color"></div>

Here is a working fiddle of that

https://jsfiddle.net/BoyWithSilverWings/ds9x55z7/