The concat() method in JavaScript creates a new array that combines the elements of the original array and the elements passed as arguments, without modifying the original array. The other methods, such as splice(), push(), and pop(), directly modify the original array.

The fs.createReadStream method in Node.js is particularly useful when dealing with large sized files. This method allows you to efficiently read large files in smaller chunks, which can be memory-efficient for handling large datasets.

When both the callback queue and the task queue are not empty, the Event Loop in Node.js follows a specific order. It first processes callbacks from the callback queue before moving to tasks from the task queue. This order ensures that callback functions, which often include I/O operations, are handled promptly.

In JavaScript, when a function is defined inside another function, it forms a "closure." A closure allows the inner function to access the outer function's variables even after the outer function has finished executing.

In a scenario with heavy Read and Update operations, sharding is a database design strategy where you distribute the data across multiple servers or nodes. This helps balance the load and improve Read performance. Sharding ensures data consistency by managing data distribution and replication across shards. Replication (Option 2) can improve Read performance but doesn't address balancing the load directly. Denormalization (Option 3) can help with Reads but may complicate data consistency. Indexing (Option 4) helps with Read performance but doesn't balance the load.

Object keys in JavaScript are case-sensitive. This means that keys with different letter casing (e.g., 'key' and 'Key') are considered distinct in an object. The other options do not accurately describe the behavior of object keys.

To handle asynchronous errors with async/await, you should use a try/catch block around each individual await operation. This allows you to catch and handle errors for specific asynchronous operations within the async function. Option A would catch errors for the entire function, which might not provide fine-grained error handling. Options C and D are incorrect approaches for error handling with async/await.

In systems with high-frequency Read operations, caching the database can optimize performance by reducing the I/O operations on the database server. Cached data can be quickly retrieved from memory, reducing the load on the underlying database.

To route requests based on the subdomain in Express.js, the most common approach is to use middleware (Option 1) that parses the subdomain from the request and then routes the request accordingly. This allows for flexibility and maintainability. Creating separate applications for each subdomain (Option 2) can lead to code duplication and complexity. Defining subdomain-specific routes using regular expressions (Option 3) can be error-prone and less maintainable. Using query parameters (Option 4) is not a common or recommended way to handle subdomain routing.

The Client Credentials grant type in OAuth 2.0 is suitable for machine-to-machine (M2M) applications where there is no user involved. It allows an application to directly authenticate itself and obtain an access token. The other grant types involve user interaction and are not intended for M2M scenarios.