What is lazy loading?
Lazy loading is a design pattern in web development that only loads necessary code components immediately, deferring nonessential components. This approach aims to improve an application's initial load time and overall performance by only loading the functionality when a user needs it.
Deferring nonessential components also enables the core functionality to load more efficiently. Lazy loading contrasts with eager loading, which loads all data immediately, even when it's not immediately required.
Lazy loading is common in web-based applications, mobile apps and software systems where initial load time and responsiveness are critical or that contain large quantities of content.
One example of lazy loading is when the user scrolls to a specific section of a web page, clicks on a button or navigates to a new page. The application waits for the user to take these actions to load necessary resources, such as images and JavaScript.
Applications can use lazy loading for images, videos, JavaScript and Cascading Style Sheets (CSS) files to improve the experience on applications that have infinite scrolling or a large amount of content, such as social media feeds or e-commerce product listings. For example, on an e-commerce site, the page waits to load certain elements until the user clicks the Load more button. In the case of infinite scrolling, the application defers resources until the user scrolls to them.
Why is lazy loading important?
End users directly experience the performance benefits of lazy loading. It helps application providers meet users' expectations for fast, responsive applications.
The development and IT teams responsible for developing, implementing and maintaining software systems also benefit from lazy loading's reduced resource consumption. When applications need a large volume and diversity of data from cloud systems, web apps, video streaming and IoT devices, organizations shift processing closer to the edge instead of relying just on centralized data centers. By managing the amount of data transferred to edge devices, lazy loading can reduce the strain on network infrastructure and cloud resources. This is particularly beneficial for mobile applications, where latency and bandwidth constraints can be an issue.
How lazy loading works
The general process of lazy loading on a web page works is as follows:
- The web page initially loads only the essential content and resources, such as Hypertext Markup Language (HTML), CSS and some JavaScript.
- As the user interacts with the page, the page detects when additional content or resources are needed and dynamically loads them.
- The web page displays loaded content or resources to the user.
Consider the following line of HTML for a lazy-loaded image:
<img src="placeholder.jpg" data-src="placeholderdata.jpg" alt="Placeholder image" loading="lazy" width="300" height="200">The line loading="lazy" is a browser attribute for lazy loading. It ensures that the image in the code only loads when it appears in the user's viewport.
Benefits and challenges of lazy loading
The main benefits of lazy loading include the following:
- Improved page load times. By deferring nonessential resources, lazy loading can significantly reduce the initial load time of a web page.
- Reduced server load. Lazy loading can help reduce the server load by only serving the essential resources, rather than loading every resource upfront. This reduced strain on the application's infrastructure can translate into cost savings and improved scalability.
- Reduced bandwidth usage. Users who don't interact with certain parts of the page won't have to download the associated resources, reducing data usage and cost savings for the end user.
- Improved user experience. Faster page load times and fewer initial resource requirements can create a more responsive user experience, which can result in better engagement and reduce bounce rates.
Lazy loading also poses several challenges to users and developers:
- Complexity. Lazy loading can add complexity because it requires additional code to detect when elements come into view and trigger loading.
- Compatibility. Lazy loading techniques might not be supported equally across all browsers. Developers should ensure that the technique works for all target users. When implemented incorrectly, lazy loading can result in a delayed display of content, disrupting the user experience.
- Indexing. Search engines might have difficulty indexing lazy-loaded content, as the initial page load might not include all the relevant content. Teams should carefully implement and monitor lazy-loaded applications to ensure page search engine optimization works as intended.
Use cases for lazy loading
Lazy loading has many use cases. Some examples include the following:
- Media. One of the most common use cases for lazy loading is the deferment of image and media assets.
- Infinite scrolling. Lazy loading helps implement infinite scrolling or pagination on web pages. As the user scrolls down the page, new content loads on-demand, rather than loading all at once.
- JavaScript and CSS. Lazy loading works for JavaScript and CSS files. Instead of loading all the scripts and styles at once, they load only when they are needed, such as when the user interacts with the corresponding part of the page.
- Third-party widgets. Web pages often include third-party widgets, such as social media buttons, comments sections or analytics scripts. Lazy loading these external resources defers them until the user accesses them or the page otherwise needs them.
- Dynamic content. Applications can lazy-load data retrieved from an API or server-side content. This helps to improve responsiveness and reduce the load on the server.
- Large data sets. Applications that use large data sets, where only a section of the total data appears at one time, can use lazy loading to only surface data the user needs to see.
