How Node.js works with its event loop and non-blocking I/O, let’s break down the process step by step:

Event Loop:

1. Initiation:
   • When you run a Node.js program, the runtime environment initializes the event loop.
   • Node.js sets up the initial context, which includes the global scope, built-in modules, and other necessary resources.
2. Execution:
   • Node.js starts executing the code in your program.
   • Asynchronous operations (e.g., reading from a file, making a network request) are initiated using callbacks, promises, or async/await syntax.
   • These operations are non-blocking, meaning the program doesn’t wait for them to complete.
3. Event Queue:
   • When an asynchronous operation is started, its associated callback function is placed in the event queue.
   • The event queue is a data structure that holds these callback functions until they’re ready to be executed.
4. Event Loop Iteration:
   • The event loop runs in an iterative cycle.
   • In each iteration, the event loop checks if there are any tasks in the event queue that are ready to be executed.
5. Callback Execution:
   • If a task’s callback is in the event queue and is ready to be executed (its associated event has occurred or its dependencies have been met), the event loop picks up the callback for execution.
6. Executing Callbacks:
   • The chosen callback function is executed. It processes the result of the asynchronous operation and performs any necessary actions.
7. Concurrency:
   • While the event loop is executing a callback, the program can continue running other code, including initiating more asynchronous operations or executing other callbacks.
8. Completion:
   • Once the callback is finished, its associated event is processed, and the callback’s execution context is cleaned up.
9. Event Loop Continues:
   • The event loop proceeds to the next iteration, checking the event queue for more tasks to execute.
   • This cycle continues as long as there are tasks in the event queue.

Non-Blocking I/O:

1. I/O Request:
   • When an I/O operation (e.g., reading a file) is requested in a Node.js program, the request is sent to the underlying operating system.
2. Background Operation:
   • The operating system starts performing the requested I/O operation in the background.
   • During this time, the Node.js program doesn’t wait for the operation to complete and can continue executing other code.
3. Callback Registration:
   • You provide a callback function that will be called when the I/O operation completes.
   • This callback function is attached to the I/O operation, effectively saying, “When you’re done, call this function.”
4. Event Queue:
   • Once the I/O operation is complete, the operating system places the associated callback function into the event queue.
5. Event Loop:
   • As explained earlier, the event loop iterates through the event queue, picking up the callback for execution when its associated event is ready.
6. Callback Execution:
   • The callback function is executed, processing the result of the I/O operation.

By combining the event loop and non-blocking I/O, Node.js can handle multiple tasks concurrently without waiting for any single task to complete. This approach is particularly effective in scenarios where an application needs to handle numerous connections, real-time interactions, and asynchronous operations simultaneously, leading to improved responsiveness and scalability.