Explain Attributes in Express.js?

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The app.listen() function is used to bind and listen the connections on the specified host and port. This method is identical to Node’s http.Server.listen() method.

If the port number is omitted or is 0, the operating system will assign an arbitrary unused port, which is useful for cases like automated tasks (tests, etc.).

The app returned by express() is in fact a JavaScript function, designed to be passed to Node’s HTTP servers as a callback to handle requests. This makes it easy to provide both HTTP and HTTPS versions of your app with the same code base, as the app does not inherit from these (it is simply a callback).