In Node.js, it’s primarily the heavy I/O tasks that are offloaded to the thread pool. Here’s a breakdown to clarify the concepts:

1. Synchronous Blocking I/O:

• Definition: Synchronous blocking I/O operations are those that stop the execution of the program until the operation is complete. For example, using fs.readFileSync() will block the event loop until the file is read.

• Behavior: These operations are not offloaded to the thread pool. They are executed directly in the main thread and block the event loop, making them less efficient for handling concurrent requests.

2. Heavy I/O Tasks:

• Definition: These include operations that can take a considerable amount of time, such as reading large files, performing complex cryptographic computations, or waiting for external resources.

• Behavior: Heavy I/O tasks are typically offloaded to the thread pool to prevent them from blocking the event loop. This allows the application to remain responsive and continue handling other asynchronous tasks while the heavy I/O operation is being processed in the background.

Summary:

• Synchronous blocking I/O operations (like fs.readFileSync()) are executed in the main thread and block the event loop; they are not offloaded to the thread pool.

• Heavy I/O tasks, on the other hand, are offloaded to the thread pool to avoid blocking the main event loop, allowing for better scalability and performance in Node.js applications.

In practice, it’s best to use asynchronous (non-blocking) I/O methods (like fs.readFile()) for I/O operations in Node.js to take full advantage of its non-blocking architecture.