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HTML

              
                	<p>This example shows a pure JS (no jQuery or shit) function that loads the images only they are currently in the visible area of the window.</p>
	<p>You can open your dev-tools and monitor how the requests are made gradually when you scroll.</p>
	<p>Note 1: only vertical scrolling is monitored here.<br/>
	Note 2: for rendering optimization, images start to load 500px under the visibility line and 50px above. You can edit this if you want to.</p>

		<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&1" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&2" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&3" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&4" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&5" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&6" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&7" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&8" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&9" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&11" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&12" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&13" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&14" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&15" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>
	<img data-src="https://picsum.photos/1000/800/?random&16" class="lazyload" width="1000" height="800" alt="random dummy image" />
	<hr/>


              
            
!

CSS

              
                img {
	max-width: 100%;
	height: auto;
	box-shadow: 5px 5px 50px black;
}

hr {
	margin: 200px auto;
}

body {
	max-width: 1000px;
	margin: auto;
}


              
            
!

JS

              
                	window.addEventListener("scroll", function() { onScrollDiv() });
	window.addEventListener("DOMContentLoaded", function() { onScrollDiv() });

	function onScrollDiv() {
		var images = document.querySelectorAll('.lazyload');
		for (var i=0, nb=images.length ; i <nb ; i++) {
			var img = images[i]
			var rect = img.getBoundingClientRect();
			var isVisible = ((rect.top - window.innerHeight) < 500 && (rect.bottom) > -50 ) ? true : false ;

			if (isVisible) {
				if (!img.src) {
					img.src = img.dataset.src;
				}
			}
		}
	}

              
            
!
999px

Console