To truly understand how JavaScript handles *concurrency*, you need to dive into its **execution model**, particularly the **event loop**. Unlike many other programming languages that use *multi-threading* for concurrency, **JavaScript is single-threaded**. This means it can execute only one piece of code at a time. However, it uses clever mechanisms to simulate *asynchronous behavior*—and that’s where the **event loop** comes in. At its core, JavaScript runs in what's called the **Call Stack**. Think of this as a stack of operations—each function that’s called gets pushed onto the stack, and when it finishes executing, it gets popped off. If a function calls another function, the new one goes on top. Easy. But here’s the twist: JavaScript often interacts with *asynchronous operations* like timers, network requests, or DOM events. You’d expect these to block execution, but they don’t. That’s because JavaScript works with something called the **Web APIs**, provided by the browser or runtime (like Node.js), to handle these operations outside the main thread. Let’s take a small example: ```javascript console.log('Start'); setTimeout(() => { console.log('Timeout'); }, 0); console.log('End'); ``` You might think the output is: ``` Start Timeout End ``` But what actually prints is: ``` Start End Timeout ``` Why? Because `setTimeout` doesn’t run immediately. It hands off the callback to the browser’s Web API, and once the timer finishes (even if it’s 0ms), the callback is placed into a **task queue**. Now enters the **event loop**. The **event loop** constantly checks: *“Is the call stack empty?”* If yes, it pushes the next message (like our `setTimeout` callback) from the queue into the stack to execute. This is how JavaScript achieves **non-blocking concurrency** even with a single thread: by **delegating tasks** and using the **event loop** to pick up completed tasks at the right time. Promises and async/await also play a huge role. They don’t use the same queue; instead, they’re handled by a **microtask queue**, which is given priority over the regular task queue. Here's an example: ```javascript console.log('A'); Promise.resolve().then(() => { console.log('B'); }); console.log('C'); ``` The output is: ``` A C B ``` Even though the promise resolves immediately, it still goes to the **microtask queue**, which runs **after the current call stack** is cleared but *before* any regular tasks like `setTimeout`. So in short, JavaScript’s **event loop**, in combination with **Web APIs**, the **task queue**, and the **microtask queue**, allows it to efficiently manage concurrency without using threads. It creates the illusion of parallelism while ensuring that your code remains predictable and consistent in behavior. Understanding this model is key to mastering **asynchronous programming** in JavaScript—and once you get the hang of it, tools like `async/await` and Promises start to make a lot more sense.