QUESTION: Что такое Event Loop?

Event Loop, or the event loop, is a fundamental mechanism in JavaScript that ensures the asynchronous behavior of the language despite its single-threaded nature. Without this mechanism, modern web development would be unimaginable, as it coordinates code execution, event handling, network requests, and interface rendering.

At the core of the Event Loop are several key components. The first of these is the Call Stack, where functions of your synchronous code are directly executed. When the interpreter encounters a function, it adds it to the stack, and removes it once execution is complete. If a function calls another one, the new function is placed on top of the first.

The second important element is the task queues. When an asynchronous operation, such as a timer or a server request, finishes, its result is sent to one of the queues. Here, there is a division into the Task Queue, which receives standard callbacks from setTimeout or DOM events, and the Microtask Queue, intended specifically for promises and DOM mutations.

The Event Loop itself is an infinite loop that constantly checks the state of the call stack. If the Call Stack is empty, the loop turns its attention to the queues. A strict priority rule applies here: the Microtask Queue is always processed before the Task Queue. The Event Loop transfers all tasks from the microtask queue to the call stack and executes them until this queue is empty. Only after that is a single task taken from the regular task queue.

This approach guarantees that promise chains and asynchronous operations will be executed as quickly as possible, preserving a predictable code execution order and preventing race conditions in a single-threaded runtime environment.