How does JS front-end framework (ember, Angular etc.) work with back-end frameworks(such as Django , Rails etc.)?

I have used django for few projects and used heroku to deploy. I have been looking into front-end frameworks such as Ember, Angular and was confused as to how it will integrate with a django project. Reading some Docs, it appears i need to have developed a REST api for my app using the backend framework in order for Javascript framework to communicate with the database. In this case the backend mostly works as a thin layer that provides access to the database. But what if I use something like parse and their Javascript API, would I even need a Backend Framework like Django?