The Event Loop

The Event Loop is the secret behind JavaScript's asynchronous, non-blocking nature. Although JavaScript itself is single-threaded (it has only one call stack), the runtime environment (like a browser or Node.js) provides additional threads for handling long-running operations, which are managed by Web APIs. Here's how it works: the Call Stack is where JavaScript code is executed. When you call a function, it's pushed onto the stack. When the function returns, it's popped off. If you call an asynchronous function like `setTimeout` or `fetch`, the operation is handed off to the Web API. The JavaScript engine doesn't wait; it immediately pops the function call off the stack and continues executing the next line of code. When the Web API finishes its task (e.g., the timer expires), it doesn't push the result back to the call stack directly. Instead, it places the callback function into the Message Queue (or Task Queue). The Event Loop is a constantly running process that has one simple job: it checks if the Call Stack is empty. If it is, it takes the first message from the Message Queue and pushes it onto the Call Stack for execution. This continuous cycle allows JavaScript to handle I/O and events efficiently without ever freezing the main thread.