If an error is thrown inside an async function and is not caught within the function, it will propagate to the global error handler, which can potentially crash the application. Proper error handling is crucial in async code.

In Express.js, you can define optional route parameters by using the ? symbol after the parameter name, like /route/:param?. This makes the parameter optional, allowing it to match both routes with and without that parameter. The other options do not represent the correct way to define optional route parameters in Express.js.

In a system with eventual consistency, Read operations may return stale or outdated data for a period of time after a Write operation. This is because eventual consistency prioritizes availability and performance over strict consistency. Strong consistency (Option 1) ensures that all reads return the most recent write but may lead to higher latency. Options 3 and 4 are not characteristic of eventual consistency.

Encryption is a security practice that involves encoding information in a way that only authorized parties with the decryption key can access it. This is commonly used to protect sensitive data during transmission and storage.

In complex CORS scenarios, you can selectively allow or deny requests by creating custom middleware on your server. This middleware can inspect the request headers, methods, or other criteria to determine whether to allow or deny a request. The other options are components of CORS but do not provide fine-grained control over request types.

The fs.watch method relies on polling on some platforms to detect file and directory changes. This polling approach may not be consistent across platforms and can have performance implications.

In Express.js, the authorize method is commonly used to protect routes and ensure that only authenticated users can access them. This is often achieved through middleware functions that check for authentication and authorization before allowing access. The other options may not be standard methods for this purpose in Express.js.

In Express.js, the app.use(express.static()) method is used to serve static files. This method allows you to specify a directory from which Express should serve static assets like images, CSS, and JavaScript. The other options are not valid methods for serving static files in Express.

To balance the use of indexing in a database with heavy read and write operations, it's crucial to use Covering Indexes for frequently queried columns. These indexes include all the necessary data for the query, reducing the need to access the main table. Implementing Partitioned Tables can isolate write-intensive data from read-intensive data, improving write performance. Adding indexes to all columns and regularly rebuilding all indexes can lead to unnecessary overhead and hinder write performance.

To handle the rejection of a Promise, you should use the catch() method. It allows you to specify what to do when the Promise is rejected with an error. The other options are not standard methods for handling Promise rejection in JavaScript.

When connecting to a NoSQL database like MongoDB in Node.js, you typically get a MongoClient object. This object represents the database connection and allows you to perform various database operations. The other options are not related to MongoDB connections.

HTTP status codes are essential in the response of an Express API. They indicate the type of response and guide client behavior. For example, a 200 OK status indicates success, while a 404 Not Found status indicates a resource was not found. Proper status codes enhance communication and handling between the client and server.