If an error is not handled by custom error-handling middleware in Express, it is handled by the default Express error handler, which is often referred to as the "defaultErrorHandler." This handler sends an error response to the client and logs the error details.

When the spread operator (...) is used on objects with the same properties, it merges the properties, with the values of the second object overwriting the values of the first object. This behavior is known as object property spreading.

When using Promise.allSettled, the status property of each object in the returned array can either be 'fulfilled' or 'rejected,' depending on the outcome of the individual Promises in the input array.

Properties added to the global object in a Node.js module can be accessed from anywhere in the module, making them globally accessible within that module.

Test coverage measures the percentage of code executed by tests. It helps identify untested code paths, improving code quality. However, it does not guarantee a bug-free codebase, as it depends on the quality of the tests. The other options provide inaccurate definitions or misconceptions about test coverage.

When creating a custom error class in Express.js, it should extend the built-in Error class. This allows you to leverage the error handling capabilities provided by JavaScript and Express.js. Extending other classes like HttpException, AppError, or CustomError is not a standard practice for custom error classes in Express.js.

In the http module, the close event of the server object is emitted when the server closes. This event allows you to perform cleanup or take action when the server is shutting down. The other options (end, disconnect, terminate) do not represent the server closing event in the HTTP module.

The for...of statement in JavaScript creates a loop that iterates over array-like objects, which include arrays, strings, maps, sets, and other objects that have iterable properties. It's particularly useful for iterating over the values of such objects.

In Node.js, the Event Loop operates on a single Core, not a thread or process. This single-threaded event loop design is one of the key features of Node.js, making it suitable for I/O-bound tasks as it can efficiently handle asynchronous operations without the overhead of multiple threads or processes.

The concept of defining a blueprint for the data in the database is known as "Schema." A schema defines the structure, relationships, constraints, and integrity rules of the data stored in a database.