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");
? 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
? How the Event Loop Works
-
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.
-
Web APIs
- Asynchronous tasks (like setTimeout) are handled here. They wait in the background.
-
Callback Queue
- Once the asynchronous task finishes, its callback is added to the queue.
-
Event Loop
- The event loop checks:
- Is the call stack empty?
- If YES, it takes tasks from the callback queue and pushes them to the stack.
- The event loop checks:
? Example: Step-by-Step
console.log("Start");
setTimeout(() => {
console.log("Timeout Task");
}, 2000);
console.log("End");
1?? Call Stack
| Step | Call Stack | Notes |
|---|---|---|
| 1 | console.log | Logs "Start" |
| 2 | setTimeout | Registers timeout task |
| 3 | console.log | Logs "End" |
2?? Web APIs
- setTimeout moves to Web APIs and starts the timer.
3?? Callback Queue
- After 2000ms, the callback (() => console.log("Timeout Task")) moves to the queue.
4?? 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
? Visualizing the Event Loop
To truly understand the event loop, check out these resources:
- Loupe
- MDN: Concurrency Model
Happy coding! ?
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