Single page applications (SPAs)

Because of this increased performance, developers can now build feature-rich JavaScript applications that run on the browser. Google was the first major company to take advantage of this, when they released the first client-side web application framework—Angular - on 20 October 2010. Since then, many competitors have emerged, including Ember, React, and Vue.js, but Angular still remains relevant today.

Angular is a framework for building SPAs. Instead of delegating the bulk of the processing to the server, the client takes on most of the responsibility.

Let's take an e-commerce web application as an example. In the client-server model, when the server receives a request from the client, it will compose a fully-formed HTML and attach it as the payload of the response. If it needs data from the database, it will query the database and inject the data into an HTML template to produce the fully-formed HTML. The client, usually a browser, is then entrusted with the simple task of rendering the HTML onto the screen.

In the SPA model, the server would initially send the entire application, including any HTML, CSS, and JavaScript files, to the client. All the application logic, including routing, now resides on the client. Because of this, the client can update the UI of the application very quickly, as it does not need to wait for a response from the server. Whenever the client requires information it does not have, such as certain entries in the database, it will send a request to the server. The server would then respond with the raw data, usually presented in JSON format, and nothing else. It is then the client's job to process this information and update the UI appropriately. With SPAs, most of the logic is handled client-side; the server's job is simply to retrieve and send back data:

The SPA model has many benefits over the client-server model:

• It frees up the server to handle more requests, as requests are simpler to process.
• It allows the UI of the app to respond more quickly to user interaction because the UI does not need to wait for the server to respond before updating itself.

Nowadays, most web applications are built with an SPA framework. Tesla, Sony, Microsoft Support, Genius, Renault, Staples, Udemy, and Healthcare.gov are all websites built with Angular; Airbnb, Asana, BBC, Dropbox, Facebook, Lyft, Netflix, PayPal, and Uber all use React on their websites; although Vue.js is relatively new, several major Asian companies have already adopted it, such as Alibaba, Baidu, Tencent, Xiaomi, and Line.