An SPA is a web app that loads a single HTML page that is dynamically updated by JavaScript as the user interacts with the app. Imagine a simple sign-up form where a user can enter a name and an email address. Once the user fills out and submits the form, a whole page refresh doesn't occur. Instead, some JavaScript in the browser handles the form submission with an HTTP POST request and then updates the page with the result of the request. Refer to the following diagram:

So, after the first HTTP request that returns the single HTML page, subsequent HTTP requests are only for data and not HTML markup. All the pages are rendered in the client's browser by JavaScript.

So, how are different pages with different URL paths handled? For example, if I enter https://qanda/questions/32139 in the browser's address bar, how does it go to the correct page in the app? Well, the browser's history API lets us change the browser's URL and handle changes in JavaScript. This process is often referred to as routing and, in Chapter 4, Routing with React Router, we'll learn how we can build apps with different pages.

The SPA architecture is what we are going to use throughout this book. We'll use React to render our frontend and ASP.NET Core for the backend API. 

Now that we have a basic understanding of the SPA architecture, we'll take a closer look at a SPA-templated app that Visual Studio can create for us.