What is the Event Loop in JavaScript?

JavaScript is a single-threaded language, meaning it executes one task at a time. But how does it handle asynchronous operations like setTimeout, fetch, and Promise without blocking the main thread? The answer lies in the Event Loop.

How the Event Loop Works

The Event Loop is a mechanism that allows JavaScript to perform non-blocking operations by offloading tasks to the browser (or Node.js) APIs and processing them in a loop.

Key Components:

1. Call Stack – Tracks function execution (LIFO: Last In, First Out).
2. Web APIs – Browser-provided APIs (setTimeout, fetch, DOM events).
3. Callback Queue (Task Queue) – Holds asynchronous callbacks.
4. Microtask Queue – For Promise callbacks and MutationObserver.
5. Event Loop – Continuously checks the call stack and queues.

Event Loop Execution Flow

1. Synchronous code runs first, filling the call stack.
2. Async tasks (like setTimeout) are sent to Web APIs.
3. Once Web APIs complete, callbacks move to the Callback Queue (macrotasks) or Microtask Queue (promises).
4. The Event Loop checks:
   • If the call stack is empty, it processes Microtasks first (higher priority).
   • Then, it picks one Macrotask (e.g., setTimeout).

Example: setTimeout vs. Promise

console.log("Start");  

setTimeout(() => console.log("Timeout"), 0);  

Promise.resolve().then(() => console.log("Promise"));  

console.log("End");  

Output:

Start  
End  
Promise  
Timeout  

Why?

• Promise (microtask) runs before setTimeout (macrotask).

Why is the Event Loop Important for Interviews?

• Explains asynchronous behavior in JavaScript.
• Helps optimize performance by avoiding blocking operations.
• Key for debugging race conditions and task scheduling.

Conclusion

The Event Loop ensures JavaScript remains non-blocking while handling async operations efficiently. Mastering it helps in writing optimized code and acing interviews.