How to load images dynamically (or lazily) when us

Question

I've noticed this in numerous "modern" websites (e.g. facebook and google image search) where the images below the fold load only when user scrolls down the page enough to bring them inside the visible viewport region (upon view source, the page shows X number of <img> tags but they are not fetched from the server straight away). What is this technique called, how does it work and in how many browsers does it work. And is there a jQuery plugin that can achieve this behavior with minimum coding.

Edit

Bonus: can someone explain if there is a "onScrolledIntoView" or similar event for HTML elements. If not, how do these plugins work?

Answer 1

I came up with my own basic method which seems to work fine (so far). There's probably a dozen things some of the popular scripts address that I haven't thought of.

Note - This solution is fast and easy to implement but of course not great for performance. Definitely look into the new Intersection Observer as mentioned by Apoorv and explained by developers.google if performance is an issue.

The JQuery

$(window).scroll(function() {
    $.each($('img'), function() {
        if ( $(this).attr('data-src') && $(this).offset().top < ($(window).scrollTop() + $(window).height() + 100) ) {
            var source = $(this).data('src');
            $(this).attr('src', source);
            $(this).removeAttr('data-src');
        }
    })
})

Sample html code

<div>
    <img src="" data-src="pathtoyour/image1.jpg">
    <img src="" data-src="pathtoyour/image2.jpg">
    <img src="" data-src="pathtoyour/image3.jpg">
</div>

Explained

When the page is scrolled each image on the page is checked..

$(this).attr('data-src') - if the image has the attribute data-src

and how far those images are from the bottom of the window..

$(this).offset().top < ($(window).scrollTop() + $(window).height() + 100)

adjust the + 100 to whatever you like (- 100 for example)

var source = $(this).data('src'); - gets the value of data-src= aka the image url

$(this).attr('src', source); - puts that value into the src=

$(this).removeAttr('data-src'); - removes the data-src attribute (so your browser doesn't waste resources messing with the images that have already loaded)

Adding To Existing Code

To convert your html, in an editor just search and replace src=" with src="" data-src="

Answer 2

This Link work for me demo

1.Load the jQuery loadScroll plugin after jQuery library, but before the closing body tag.

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script><script src="jQuery.loadScroll.js"></script>

2.Add the images into your webpage using Html5 data-src attribute. You can also insert placeholders using the regular img's src attribute.

<img data-src="1.jpg" src="Placeholder.jpg" alt="Image Alt"><img data-src="2.jpg" src="Placeholder.jpg" alt="Image Alt"><img data-src="3.jpg" src="Placeholder.jpg" alt="Image Alt">

3.Call the plugin on the img tags and specify the duration of the fadeIn effect as your images are come into view

$('img').loadScroll(500); // in ms

Answer 3

Lazy loading images by attaching listener to scroll events or by making use of setInterval is highly non-performant as each call to getBoundingClientRect() forces the browser to re-layout the entire page and will introduce considerable jank to your website.

Use Lozad.js (just 569 bytes with no dependencies), which uses IntersectionObserver to lazy load images performantly.

Answer 4

Dave Artz of AOL gave a great talk on optimization at jQuery Conference Boston last year. AOL uses a tool called Sonar for on-demand loading based on scroll position. Check the code for the particulars of how it compares scrollTop (and others) to the element offset to detect if part or all of the element is visible.

jQuery Sonar

Dave talks about Sonar in these slides. Sonar starts on slide 46, while the overall "load on demand" discussion starts on slide 33.

Answer 5

Im using jQuery Lazy. It took me about 10 minutes to test out and an hour or two to add to most of the image links on one of my websites (CampusCube). I have NO (none/zero) relationship of any kind to the dev, but it saved me a lot of time and basically helped improve our bounce rate for mobile users and I appreciate it.

Answer 6

The Swiss Army knife of image lazy loading is YUI's ImageLoader.

Because there is more to this problem than simply watching the scroll position.