In Express.js, you can completely disable built-in middlewares if they are not needed, allowing for customization and control over the middleware stack.

The pop() method in JavaScript is used to remove the last element from an array and return that element. It modifies the original array by removing the last element. The other options (shift(), splice(), and unshift()) are used for different array operations.

The forEach method is used in JavaScript to iterate over all enumerable properties of an object. It is commonly used with arrays to perform an action on each item. The for loop and while loop are general looping constructs and not specific to object iteration. iterate is not a standard method.

The "aud" (audience) claim in a JWT (JSON Web Token) represents the intended audience for which the JWT is intended. It specifies the recipients or systems that are expected to process the token. The "iss" (issuer) claim (Option 2) identifies the entity that issued the token. The "exp" (expiration) claim (Option 3) indicates the time after which the token should not be accepted. The "sub" (subject) claim (Option 4) typically identifies the subject of the token, often the user or system the token represents.

Mocking in unit testing is primarily used to simulate the behavior of external dependencies, such as databases, APIs, or services, so that the unit being tested can be isolated and tested in isolation. This helps ensure that tests focus on the unit's logic without relying on the actual external dependencies.

The spread operator can be used to merge two "Objects" into a new one, combining their properties. It's a useful way to create a new object with properties from multiple source objects.

To create a function that accepts an arbitrary number of arguments and returns an array of those arguments, you would use the rest parameter syntax (...args) in the function's parameter list. This syntax collects all the passed arguments into an array named args. The other options are either incorrect or do not use the rest operator correctly.

The significance of 'backpressure' in streams is that it ensures that data is not lost when reading from or writing to a stream. It allows the consumer of data to control the rate of data flow, preventing buffer overflow and resource exhaustion. The other options do not accurately describe the concept of 'backpressure.'

In Express.js, specific error handlers for different error types can be created by utilizing the next(err) function with custom error classes. This allows you to define error-handling middleware that can respond to specific error types based on their custom classes. The other options are not typically used for handling specific error types in Express.js.

Non-Blocking I/O in Node.js refers to I/O operations that execute concurrently without blocking the main thread. It allows Node.js to perform multiple I/O operations without waiting for each operation to complete. The other options describe blocking I/O or do not accurately describe non-blocking I/O.

To minimize the load on the database in an e-commerce application, implementing proper indexing is crucial. Indexes allow the database to efficiently retrieve data. Denormalized schemas can lead to redundancy and maintenance challenges. While stored procedures can be useful, they are not the primary means of optimizing queries. Avoiding primary keys is not recommended; they are essential for data integrity.

To read the contents of a directory in Node.js, you should use the fs.readdir() method. It returns an array of all the file and directory names in the specified directory. The other options are not valid fs methods for this purpose.