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What is IntersectionObserver?
Contents:


  1. Five Techniques to Lazy Load Images for Website Performance — SitePoint
  2. 1. Resize your images
  3. How to deploy the lazy loading script?

In the JavaScript document, you then create a config object and register it with an intersectionObserver instance:. Fortunately, a polyfill is available. Robin Osborne suggests a super ingenious solution based on progressive enhancement.

Five Techniques to Lazy Load Images for Website Performance — SitePoint

Why progressive enhancement? Well, if you display images using a JavaScript-based solution, what happens if JavaScript is disabled or an error occurs which prevents the script from working as expected? In this case, without progressive enhancement, users are likely to see no images at all.

Not cool. A quick and easy alternative for implementing lazy loading of images is to let a JS library do most of the job for you. Lozad is a highly performant, light and configurable lazy loader in pure JavaScript with no dependencies. You can use it to lazy load images, videos, iframes, and more and it uses the Intersection Observer API.

Only, be mindful of browser support and eventually integrate this library with a polyfill for the Intersection Observer API.

1. Resize your images

If you are a Medium reader, you have certainly noticed how the site loads the main image inside a post. The first thing you see is a blurred, low-resolution copy of the image, while its high-res version is being lazy loaded:. Yall is a feature-packed lazy loading script for images, videos, and iframes.

More specifically, it uses the Intersection Observer API and smartly falls back on traditional event handler techniques where necessary. And there you have it — five ways of lazy loading images you can start to experiment with and test out in your projects. Blurred placeholder image on Medium website. High-res, lazy loaded image on Medium website. It automatically converts an image to WebP wherever possible and also optimizes the image quality in real-time.

And yes, while mobile phones have gotten powerful and mobile networks have become better, data suggests that mobile data speeds are still much slower than broadband speeds. There are countries or areas within countries where the mobile data connection is flaky. So, it is important to be extra careful when designing web experiences for mobile.

If you have a responsive website for desktop and mobile, you can switch to using responsive images. The browser then decides the best image size to load on a particular device from the available list based on the device dimensions and the layout you specify.


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Modern mobile phones have high density screens that pack more pixels in the same square inch. An image that look fine on regular devices, would look slightly blurred on a high density screen.

What Is Lazy Loading?

A solution to this is to load a 2x size image on screens with DPR 2, a 3x image on screens with DPR 3 and the normal image 1x size image on other devices. This too can be accomplished using the responsive image tag as shown below. How client hints work is a huge topic in itself that it is out of scope for this post and has been covered in detail here.

ImageKit provides you with a URL-based DPR parameters along with resize and crop parameters, and also supports client hints, that makes it very easy to use responsive images and deliver perfect images across devices. Even after you have optimized all of your images, loading too many of them is bound to slow down your website and negatively impact the user experience.

I am not advocating that we should use lesser images.

Native image lazy-loading is coming to the web! -- Faster Frontend Web Development

But, there are cases where we can avoid using images or avoid loading them up front. For example , instead of loading images, you can create buttons, gradients and other advanced elements using CSS. The other more important technique that you can use is lazy loading for your images. Lazy loading basically means that we defer loading of images that are not required immediately. Typically, any image that is not visible to the user on his screen or the viewport, can be loaded at a later point in time i. Consider a case where you have products on your web page.

How to deploy the lazy loading script?

If you request all the product images at the same time and at the very beginning, it would slow down the load time. Then, when the user starts scrolling down the page, we will keep loading more images. This would help improve the initial load time and the user experience. Thus, you also end up saving on bandwidth costs for image delivery. Getting started with lazy loading is very simple with JS libraries like jQuery Lazy. Once you have solved for the size of the images and the number of images that get loaded on a particular page, the next step is to ensure that the images that do get loaded on your website, are loaded quickly.

Decreasing the image load time will not only help you get a faster overall page load time and thus a better the user experience on your website, but would also help you rank higher on search engines. This article by Incapsula does a good job of explaining how a CDN works. Then, if a user from Brazil requests an image from your website, instead of getting that image from your server in USA, the CDN delivers it from a node closest to that user in Brazil. This cuts down the round trip time needed to load an image. Some of the notable CDNs are listed on this Wikipedia page.

It uses techniques like multiplexing, header compression and server push to reduce page load time.

Without any extra effort, you can get the best-in-class delivery for your images and even other static files using ImageKit. Well there are quite a few tools that you can use to test a website specifically for image related issues. One way is to use this website analyzer by ImageKit. Google has also worked on an open-source tool called the Lighthouse. This tool comes integrated in the latest versions of Chrome and can perform a thorough analysis of not just the images on your website but also other issues that might be impacting performance.

Google PageSpeed insights also points out if you are loading unoptimized images on your website apart from other recommendations. We have covered all the major techniques around image optimization and performance improvement. Always remember, for images on your website — load lighter, load fewer, and load faster. Sign in. Get started.