A JavaScript framework is a web application framework written in JavaScript. A JavaScript framework differs from a JavaScript library; a library offers multiple predefined functions that a developer can call to improve and expand their application, whereas a framework describes the structure of the application and gives the developers a way to organize their code to make their app more flexible and scalable. A developer cannot call a framework; instead it is the framework that will call and use the code in some particular way.
JavaScript frameworks are usually based on the MVC architectural pattern because they are also designed to separate the different aspects of a web application to improve the quality of code and to make the development easier.
Some famous examples of a JavaScript framework with a MVC architecture are AngularJS and Ember.js.