How does JavaScript handle asynchronous operations?

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JavaScript handles asynchronous operations using several mechanisms: callbacks, promises, and async/await.

Originally, callbacks were used—functions passed as arguments to other functions that execute after an operation completes. While effective, deeply nested callbacks can lead to "callback hell," making code hard to read and maintain.

To improve this, JavaScript introduced Promises. A Promise represents a value that may be available now, later, or never. It has three states: pending, fulfilled, or rejected. Developers use .then() to handle success and .catch() for errors, enabling cleaner chaining of asynchronous steps.

Later, async/await syntax was introduced to make asynchronous code look and behave like synchronous code. Functions declared with async automatically return a promise, and await pauses execution until the promise resolves or rejects. This simplifies error handling using try/catch.

Under the hood, JavaScript uses the event loop and a message queue. Asynchronous tasks (e.g., timers, HTTP requests) are processed outside the main thread. Once completed, their callbacks are queued and eventually executed when the call stack is clear, ensuring non-blocking execution.

This event-driven model allows JavaScript, especially in browsers and Node.js, to remain responsive and efficient, even when handling I/O-heavy tasks.

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