The four components of the MEAN stack were developed at different points of time. But, now they are assembled together to form the MEAN stack. It provides a modern approach which simplifies and stimulates web application development.
MongoDB is classified as a NoSQL, schemaless database system. It stores data in JSON format which makes it easier to pass data between client and server. The move from a traditional table-based relational database structure to a NoSQL, such as MongoDB represents JSON-like documents with dynamic schemas. MongoDB is a free and open-source cross-platform document-oriented database.
Express.js is a minimal and flexible web framework to build web applications that runs on Node JS. It still provides an elegant set of features to deal with including single and multi page, hybrid web applications, the RESTful API. Express reduces trouble in setting up a server to handle incoming requests and return relevant response, server side rendering, routing for large-scale applications.
It's objective is to build up browser-based applications with Model-View-Controller(MVC) capacity, in an intention to make both development and testing easier.
AngularJS runs uniformly well on desktops and laptops, smartphones and tablets, and even smart TVs. And testing is no longer a second thought with best world-class end-to-end testing frameworks such as KarmaJS and JasmineJS, using these frameworks you can write accurate test suites for your MEAN app.
MEAN makes use of the strength of modern and productive single and multi page applications, which does not need to reload a web page for every single request to server like the most traditional web applications do. By using a uniform language and form a full-stack solution. MEAN stack is a very simple way to create a responsive, modern and dynamic web applications.