Lazy Loading in Web Applications: JavaScript, Angular, and React

Lazy Loading in Web Applications: JavaScript, Angular, and React

Get a primer on the concept of lazy loading, how to implement it in pure JavaScript, and use the built-in functionality in Angular and React.

Get a primer on the concept of lazy loading, how to implement it in pure JavaScript, and use the built-in functionality in Angular and React.

Lazy loading is a common technique to improve performance in web applications. The concept is simple—instead of loading everything upfront, leading to the annoying “loading” message experienced by many users of single page applications—the application loads components only when they are actually requested or needed by the user.

Lazy loading is supported natively in many web development frameworks. In this article, I discuss the concept of lazy loading and how to implement it in pure JavaScript, and using built-in functionality in Angular and React. Finally, I provide a quick tutorial showing a real life implementation of lazy loading in a single page application.

What is Lazy Loading?

Lazy loading (also known as dynamic function loading) allows developers to specify program components that should not be loaded by default when the program starts. Normally, the system loader automatically loads the initial program and all dependencies. When using lazy loading, dependencies are only executed when explicitly needed.

If users don’t initially use some of the dependent components, lazy loading can significantly improve performance. The downside of lazy loading is that function calls to lazy loading components require extra guidance and time. So before embarking on a lazy loading journey, consider how much of a performance improvement it can provide.

The concept of lazy loading has been widely used in web development frameworks. Most commonly, it is used to delay loading of rich media—images, video, and dynamic elements—that are below the fold and initially not visible to website users. For long, complex web pages or interface screens, lazy loading images and video can dramatically improve page load time, which can boost user engagement, conversion, and search ranking.

Lazy loading works differently in different frameworks:

  • In pure JavaScript, there are simple scripts that can help you load images when they enter the user’s viewport, using the Intersection Observer API supported by most major browsers.
  • In Angular, there are several libraries that can help you lazily load NgModules, and you can also configure lazy loading natively using Routes.
  • In React, you can use the built-in React.lazy() method to delay loading of components, or wrap multiple components for lazy loading using the Suspense component.

javascript angular react web-development

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