Single Page applications are built with one HTML Page and is loaded from server once into the browser. They continue to live on browser. Different screens are navigated as part of Javascript. Any data is fetched with the REST api from the Javascript code.
Since Single page applications load the entire application code into the browser, the performance is better. Hence they are more aptly called Client side apps. They are in contrast to server side web application where every screen is loaded from server into the browser.
Many technologies support building Single page applications inluding Angular, React etc.. Since the entire code lives on client side, we have to be careful in deciding what information can be stored in client side code. For example, secure client secrets are not advisable to be stored in client side apps.
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