MongoDB, Express, React, and Node are all used in tandem to build web applications and make up the MERN stack. In this lineup, Node and Express bind the web backend together, MongoDB serves as the NoSQL database, and React makes the frontend that the user sees and interacts with.

All four of these technologies are free, open source, cross-platform, and JavaScript-based, with extensive community and industry support. Each technology has a unique set of attributes, which, when integrated together, make a simple but effective full JavaScript stack for web development.

Since these are independent technologies, it is also important to recognize these as moving parts in your project that need to be configured, combined, and extended with additional parts to meet the specific requirements of your project. Even if you are not an expert in all the technologies in this stack, you need familiarity with each and an understanding of how these can work together.

Node was developed as a JavaScript runtime environment built on Chrome's V8 JavaScript engine. Node made it possible to start using JavaScript on the server side to build a variety of tools and applications beyond previous use cases that were limited to being within a browser.

Node has an event-driven architecture capable of asynchronous, non-blocking I/O (short for Input/Output). Its unique non-blocking I/O model eliminates the waiting approach to serving requests. This allows you to build scalable and lightweight real-time web applications that can efficiently handle many requests.

Node's default package management system, the Node Package Manager or npm, comes bundled with the Node installation. npm gives you access to hundreds of thousands of reusable Node packages built by developers all over the world and boasts that it is currently the largest ecosystem of open source libraries in the world.

However, npm isn't the only package management system at your disposal. Yarn is a newer package manager developed by Facebook and has been gaining popularity in recent years. It can be used as an alternative to npm, with access to all the same modules from the npm registry and more features that are not yet available with npm.

Node will enable us to build and run complete full-stack JavaScript applications. However, to implement an extensible server-side application with web application-specific features such as API routing, we will use the Express module on top of Node.

Express is a simple server-side web framework for building web applications with Node. It complements Node with a layer of rudimentary web application features that provide HTTP utility methods and middleware functionality.

In any web application developed with Node, Express can be used as an API routing and middleware web framework. It is possible to insert almost any compatible middleware of your choice into the request handling chain, in almost any order, making Express very flexible to work with.

In the MERN-based applications that we will develop, Express can be used to handle API routing on the server side, serve static files to the client, restrict access to resources with authentication integration, implement error handling, and, essentially, add on any middleware package that will extend the web application functionality as required.

A crucial functionality in any complete web application is the data storage system. The Express module does not define requirements or put restrictions on integrating databases with a Node-Express web application. Therefore, this gives you the flexibility to choose any database option, be it a relational database such as PostgreSQL or a NoSQL database such as MongoDB.

MongoDB is a top choice when deciding on a NoSQL database for any application. It is a document-oriented database that stores data in flexible, JSON-like documents. This means that fields can vary from document to document and data models can evolve over time in response to changing application requirements.

Applications that place a high priority on availability and scalability benefit from MongoDB's distributed architecture features. It comes with built-in support for high availability, horizontal scaling using sharding, and multi-data center scalability across geographic distributions.

MongoDB has an expressive query language, enabling ad hoc queries, indexing for fast lookups, and real-time aggregation that provides powerful ways to access and analyze data while maintaining performance even when data size grows exponentially.

Choosing MongoDB as the database for a Node and Express web application will make a fully JavaScript-based and standalone server-side application. This will leave you with the option to integrate a client-side interface that may be built with a compatible frontend library such as React to complete the full-stack application.

React is a declarative and component-based JavaScript library for building user interfaces. Its declarative and modular nature makes it easy for developers to create and maintain reusable, interactive, and complex user interfaces.

Large applications that display a lot of changing data can be fast and responsive if built with React, as it takes care of efficiently updating and rendering just the right user interface components when specific data changes. React does this efficient rendering with its notable implementation of a virtual DOM, setting React apart from other web user interface libraries that handle page updates with expensive manipulations directly in the browser's DOM.

Developing user interfaces using React also forces frontend programmers to write well-reasoned, modular code that is reusable and easier to debug, test, and extend.

Since all four technologies are JavaScript-based, these are inherently optimized for integration. However, how these are actually put together in practice to form the MERN stack can vary based on application requirements and developer preferences, making MERN customizable and extensible to specific needs. Whether this stack is a relevant option for your next full-stack web project not only depends on how well it can meet your requirements, but also on how it is currently faring in the industry and where these technologies are headed.