If your application is a bit more than a throwaway prototype or a glorified one-pager, you are likely dealing with remote APIs. These remotes APIs, in turn, are communicating with a backend layer (for example, PHP, Ruby, or Golang) and databases (for example, MySQL, MS SQL, or Oracle).
While this book focuses on Angular application, we cannot ignore the fact that they do not usually exist by themselves. Indeed, any meaningful application will need to pull and push data from/to somewhere.
With that in mind, let's imagine that your application is some sort of frontend for an online e-commerce site such as Amazon. This made-up application would certainly have a profile page where your users can see their past and ongoing commands.
Let's further specify our application by imagining that your APIs, endpoints are specified as follows:
GET /orders
This returns...