In the vast majority of cases, frameworks are used to write the so-called Single Page Applications. Those. everything that goes on the site goes on one page, without a direct transition from it.
With their help, you can develop both full-fledged sites and functional modules (various online tools). Of course, full-fledged frameworks are better suited for the first task, and for the second it is recommended to use lighter-weight frameworks or libraries.
Let’s talk a little about frameworks first. They provide a clear structure of the application and are implemented using so-called “design patterns” (a concept that has migrated more from the backend, but clearly takes place in front-end development using js frameworks). The most widely used patterns are MVC (Model-View-Controller), MVP (Model-View-Presenter) and MVVM (Model-View-ViewModel). There is a huge amount of information on the Internet that describes each of them in detail, and the disclosure of this topic is not the main task of the article, because whoever is interested can independently familiarize himself with these concepts.
Advantages of building an application on a JS framework:
you can easily implement SPA (Single Page Application);
using a js framework obliges us to have an application structure (let’s say a resounding “no” to spaghetti code);
the code becomes noticeably smaller and cleaner, which positively affects the speed of development, as well as the support and elimination of errors in the application code;
the presence of the structure implies the modularity of the application, and this makes it easier for several developers to work on the application at the same time;
Of the significant shortcomings, only temporarily incomplete support by search engines can be distinguished, but this task rarely coincides with the task of implementing SPA (Single Page Application), especially since the leading search engines (at least Google) have almost completely solved this problem.
A lot of applications on js frameworks have already been released and this segment is only gaining momentum. For examples of completed projects on Angular.js, for example, the whole site builtwith.angularjs.org is highlighted, lists of sites and applications using other frameworks can be found below on the main pages of the official sites: backbone.js, ember.js.