Since Express.js is considered to be a lightweight web application framework, that is it had only the basic web application building blocks built into it, it becomes necessary for it to provide a way to add additional building blocks encapsulating additional functionality that may be needed by more complex applications.

Express.js does this through what is called a middleware. In this video, we walk through exactly what middleware is, how Express.js allows the inclusion of custom-built middleware into the framework, and how to use those middleware within your application.

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