What is single-page application (SPA)?

What is a single-page application example?

Well-designed SPAs deliver unique, engaging and enhanced user experiences. This is why many organizations rely on SPAs to connect with their audiences and achieve their business goals.

Some familiar examples of SPAs include Google, Facebook, Gmail, Google Maps, Twitter, Netflix, GitHub, Airbnb and Pinterest.

In general, SPAs are best suited for websites that do the following:

• Use a lot of templates.
• Provide real-time updates to the user.
• Include interactive features like animations or forms.
• Involve dynamic user interactions and inputs.
• Require frequent data manipulations or updates.

This is why SPAs are the preferred architecture for many social media sites, e-commerce platforms, online gaming sites, banking sites and collaboration tools.

What is SPA architecture and how does it work?

The SPA architecture, like non-SPAs, involves a client (web browser) and a web server. But unlike non-SPAs, the initial GET request in SPAs results in the loading of a single, bare-bones HTML file. All subsequent client-server interactions involve changes to the webpage that are displayed when the user interacts with it, for example, by clicking on a link or submitting a form.

Many SPAs use HTML5 and AJAX (Asynchronous JavaScript and XML) to enable smooth and dynamic responses to user requests, allowing content to update immediately when a user performs an action. Once the page is loaded, interactions with the server are through AJAX calls and data is returned, typically in JSON (JavaScript Object Notation) format, to update the page without requiring reloads. JavaScript also plays a role in the form of frameworks to render the initial page or APIs to interact with server-side resources, pull in page content and functionality, and retrieve data.

Advantages of single-page applications

One of the key benefits of SPAs is that they deliver tangible performance gains compared to other applications. This is because entire pages are not loaded from the web server with each navigation or user-generated event. Rather, the content is dynamically retrieved and updated only in response to some user action. Thus, by reducing server requests and minimizing, or in many cases eliminating, page reloads, SPAs speed up load time.

Faster loading also positively impacts user experiences. The user doesn't have to wait for a page to load as they interact with the application. In this sense, the site behaves more like a desktop application. The app's efficient caching process and lower lag time between the loading of its successive pages enhance user engagement and enable a more fluid browsing experience.

SPAs also require less development time and effort compared to non-SPAs. The separation of the application's back-end (services) from the front-end (display) creates a decoupled architecture that allows the front-end and back-end teams to work independently and collaborate as needed. Further, the use of APIs allows the front-end teams to focus on developing the user interface without needing to worry about the back-end logic or data. Often, teams can also reuse the same back-end server-side code to save more time and accelerate development.

With SPAs, many functionalities can be built as microservices. This allows for easier experimentation and faster updates to the application's content, navigation, display and other aspects.

Disadvantages of single-page applications

Despite its advantages, building an SPA can require substantial coding effort. For example, developers may need to build a server-side script to manage the app's various changes through AJAX. Some additional effort may also be required to implement navigation within the app, to maintain its state, and to test and debug it across different environments prior to release.

Search engine optimization can also be difficult for SPAs, and might even negatively affect a site's SEO. This can happen when a search engine's web crawlers cannot see the SPA's content. This prevents proper indexing, which then makes it harder for users to find it via the search engine.

That said, content management systems reduce the effort and enable developers to build SPAs faster. The most useful CMSes for developing SPAs are API-based, easy to use, feature a decoupled architecture where the content layer is separated from the presentation layer, and allow personalization on the server side.

On the user side, SPAs can significantly load the browser, which can slow downloading and negatively impact the user experience. Low memory on the user's device can also affect SPA performance.

Security may be a concern for some SPAs if they were not built using secure coding best practices. Since SPAs rely on client-side rendering, they are vulnerable to cross-site scripting attacks in which threat actors inject malicious scripts into the application to get it to behave in unusual or unexpected ways. Security vulnerabilities may also expose user data to adversaries, placing them at risk of breaches, identity theft, fraud and other issues.

Finally, some organizations encounter problems when migrating from a multi-page application (MPA) to an SPA. For example, if the back-end APIs don't support Cross-Origin Resource Sharing, some of the SPA's functionality may not work. Inconsistent UX is another potential problem across different devices, screen sizes, resolutions and browsers. Data transfers may also be affected, especially if the application is meant to work with legacy systems.

SPAs vs. MPAs

MPAs present the user with an initial page that is linked to parts of the application on separate HTML pages, which means that the user must wait for a new page to load every time they make a new request. These apps, and the underlying web server and web browser, work based on a traditional client-server model.

With this model, the web browser requests the web server to send the content for a webpage. On receiving this request, the server sends back some HTML elements to the client, such as text, images, CSS and so on. This allows the user to view the webpage. Here, it's important to note that the server always sends the full HTML document on receiving a request from the client to render a webpage.

The architecture of an SPA is based on a different client-side paradigm. Thus, while an SPA may retrieve all of the HTML, JavaScript and CSS code on the initial load, it is more common to load these resources dynamically to update and display the webpage in response to user interaction or other events. Simply put, a single HTML file is loaded initially when the browser makes a request to the web server. That page is then dynamically rewritten as the user interacts with it.

Technologies used to build SPAs

Many development platforms and frameworks are available to build high-performance SPAs, specifically their client-side aspects. Popular frameworks include Angular, React and Vue.js.

Angular's component-based framework, libraries and developer tools make it easy to build SPAs using HTML and TypeScript. Since Angular can easily handle large applications, it is a popular framework choice for many companies, including Google and Netflix.

Vue.js is a flexible open source JavaScript framework for building SPAs. It features a reactive rendering system that automatically updates the user interface in response to data changes.

React is a JavaScript library that provides an extensive ecosystem of plugins and tools that developers can use to create interactive user interfaces.

Additionally, teams may use AJAX and HTML5 to build responsive, dynamically updated pages. AJAX is a set of web application development technologies that facilitate data exchange and seamless application-user interactions. HTML5 is a coding language -- specifically a markup language -- to create dynamic, interactive and responsive SPAs.

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