🎯 What is the Event Loop in JavaScript? A Complete Guide with Diagrams

🔁 What is the Event Loop in JavaScript? A Complete Guide with Diagrams

Have you ever wondered how JavaScript handles asynchronous operations like setTimeout, Promises, or fetch()? The secret lies in something called the Event Loop.

In this blog post, we’ll break down:

• ✅ What the JavaScript Event Loop is
• ✅ How the Call Stack, Web APIs, and Queues work together
• ✅ A visual diagram to understand the flow
• ✅ A practical code example to clear your doubts

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🔄 What is the Event Loop?

JavaScript is a single-threaded language, meaning it can only do one thing at a time. But it still handles asynchronous tasks efficiently thanks to the Event Loop.

Let’s understand how it works through its key components.

📊 Components of the Event Loop

• Call Stack: Where JavaScript tracks which function is currently running.
• Web APIs: Browser-provided functionalities like setTimeout, DOM Events, etc.
• Callback Queue: Holds callbacks from Web APIs waiting to be executed.
• Microtask Queue: Contains tasks like Promise.then() callbacks and runs before Callback Queue.
• Event Loop: Continuously checks the Call Stack and moves tasks from queues when the stack is empty.

🧩 Visual Diagram

🧪 Example Code

console.log('Start');

setTimeout(() => {
  console.log('setTimeout');
}, 0);

Promise.resolve().then(() => {
  console.log('Promise');
});

console.log('End');

🔎 Output:

Start
End
Promise
setTimeout

🧠 Why?

Because Promise callbacks go to the Microtask Queue and are processed before the Callback Queue, which is where setTimeout lands.

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📌 Conclusion

The JavaScript Event Loop is crucial for handling asynchronous behavior. Understanding it helps you write efficient code and debug complex asynchronous bugs.

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