The 6 Stages of the Node.js Event Loop

The event loop is a crucial mechanism in Node.js that manages asynchronous callbacks, enabling the platform to perform non-blocking I/O operations. Here’s a detailed breakdown of 6 key points of the event loop operates.

1. Call Stack
2. Callback Registration
3. Callback Queues
4. Continuous Monitoring
5. Completion of Asynchronous Operations
6. Execution Flow

1. Call Stack: Understanding the Call Stack in JavaScript:

• The call stack is where JavaScript code is executed. When a function is called, it is pushed onto the call stack. Once the function execution is complete, it is popped off the stack.
• If the call stack is busy executing functions, the event loop waits until the stack is empty before checking the callback queues.

2. Callback Registration: How Asynchronous Operations Are Handled:

• Web APIs and Node.js APIs: When an asynchronous operation (like a file read, network request, or timer) is initiated, the corresponding callback is registered with a Web API (in the browser) or a Node.js API (in the server).
• Example:
  • For setTimeout(), when the timer expires, the callback function is registered with the event loop to be executed in the macrotask queue.
  • For a network request made via http.get(), when the data is received, the callback is placed in the queue for processing.

3. Callback Queues: Macrotask vs. Microtask:
Node.js maintains two primary types of callback queues

• Macrotask Queue (Task Queue): This queue handles callbacks from operations such as setTimeout, setInterval, I/O operations, and other tasks that take time. Tasks in this queue are executed only after all microtasks have been processed and the call stack is empty.
• Microtask Queue: This queue includes callbacks from Promises and process.nextTick(). Microtasks have a higher priority and are executed immediately after the call stack is empty, before processing any macrotasks.

4. Continuous Monitoring by the Event Loop:

• The event loop continuously checks the call stack and the callback queues to determine if there are any tasks that need to be executed. This allows Node.js to handle multiple asynchronous operations concurrently without blocking the main thread.

5. Completion of Asynchronous Operations:
Once an asynchronous operation completes, the corresponding callback function is added to the appropriate queue:

• If a timer set by setTimeout() expires, the associated callback is registered in the macrotask queue, waiting to be executed after the current stack and any microtasks are processed.

6. Execution Flow: Moving Tasks Back to the Call Stack:

• When the call stack is empty, the event loop first processes all the tasks in the microtask queue. This ensures that promise resolutions and other microtasks are handled as soon as possible.
• After all microtasks are completed, the event loop will move on to the macrotask queue and execute one task at a time. This task execution continues until the stack is empty again, at which point the loop checks the queues once more.
• Once the task is executed, it is removed from the call stack

Examples of Event Loop Execution:

console.log('Start');

setTimeout(() => {
  console.log('Timeout');
}, 0);

Promise.resolve().then(() => {
  console.log('Promise');
});

console.log('End');

Output:

Start
End
Promise
Timeout

In this example:

• The synchronous logs ('Start' and 'End') execute first, adding them to the call stack.
• The setTimeout callback is registered in the macrotask queue.
• The Promise resolves, adding its callback to the microtask queue.
• After the call stack is cleared, the event loop processes the microtask queue ('Promise') before moving on to the macrotask queue ('Timeout').

The event loop is a core feature of Node.js that enables efficient management of asynchronous operations. By continuously monitoring the call stack and callback queues, it ensures that tasks are processed in a non-blocking manner. The registration of callbacks in Web APIs and Node.js APIs is key to maintaining responsiveness in applications, allowing Node.js to handle numerous concurrent operations efficiently.