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Understanding the JavaScript Event Loop (Made Simple)

Susan Sarandon
Release: 2024-12-24 11:04:11
Original
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Understanding the JavaScript Event Loop (Made Simple)

Understanding the JavaScript Event Loop (Made Simple)

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


? 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");
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? Output:

Task 1
Task 2
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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");
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? 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.

✨ Example: Step-by-Step

console.log("Start");

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

console.log("End");
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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! ?

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