The Alfresco JavaScript API is not normally used directly. Instead the Alfresco Application Development Framework (ADF) is used, which uses the JavaScript API indirectly. But there are situations when it might be necessary to use the JavaScript API directly, such as when ADF cannot be used. ADF is based on Angular and if another JavaScript library such as React has been adopted, then it is beneficial to use the Alfresco JavaScript API directly from React.
The Alfresco JavaScript library abstracts the Content Services (ACS) ReST API and the Alfresco Process Services (APS) ReST API, so a lot of work has been done to make it smooth to use the Alfresco ReST APIs from a third party JavaScript library. For example, authentication with both ACS and APS is handled automatically by the Alfresco JavaScript library.
For more information about the Alfresco JavaScript API, and examples of how to use it, have a look here.