Understanding the JavaScript Event Loop (Made Simple)

The JavaScript event loop is what makes asynchronous programming in JavaScript possible. Here's a simple explanation!

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🧠 Key Concepts

1. Single-Threaded

JavaScript can only do one thing at a time because it’s single-threaded.

console.log("Task 1");
console.log("Task 2");

👉 Output:

Task 1
Task 2

2. Synchronous vs. Asynchronous

• Synchronous tasks: Run in order, one after another.
• Asynchronous tasks: Don't block the main thread; they wait until they're ready to run.

console.log("Start");

setTimeout(() => {
  console.log("Async Task");
}, 1000);

console.log("End");

👉 Output:

Start
End
Async Task

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🔄 How the Event Loop Works

1. Call Stack

   • The place where tasks are executed one by one.
   • When a function runs, it gets added to the stack. When it finishes, it gets removed.

2. Web APIs

   • Asynchronous tasks (like setTimeout) are handled here. They wait in the background.

3. Callback Queue

   • Once the asynchronous task finishes, its callback is added to the queue.

4. Event Loop

   • The event loop checks:
     1. Is the call stack empty?
     2. If YES, it takes tasks from the callback queue and pushes them to the stack.

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✨ Example: Step-by-Step

console.log("Start");

setTimeout(() => {
  console.log("Timeout Task");
}, 2000);

console.log("End");

1️⃣ Call Stack

Step	Call Stack	Notes
1	console.log	Logs "Start"
2	setTimeout	Registers timeout task
3	console.log	Logs "End"

2️⃣ Web APIs

• setTimeout moves to Web APIs and starts the timer.

3️⃣ Callback Queue

• After 2000ms, the callback (() => console.log("Timeout Task")) moves to the queue.

4️⃣ Event Loop

• The event loop checks if the call stack is empty.
• The callback is moved to the stack and executed.

👉 Final Output:

Start
End
Timeout Task

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🚀 Visualizing the Event Loop

To truly understand the event loop, check out these resources:

• Loupe
• MDN: Concurrency Model

Happy coding! 🎉