Demystifying the JavaScript Event Loop

May 15, 2024 (10mo ago)

What is the JavaScript Event Loop?

JavaScript is single-threaded, meaning it can only execute one task at a time. But how does it handle multiple tasks, like fetching data from an API while still responding to user interactions?

Enter the event loop—a mechanism that manages asynchronous operations and ensures the execution of callbacks.

Understanding the Event Loop in Action

To see the event loop in motion, let’s analyze this code snippet:

console.log("Start");
 
setTimeout(() => {
  console.log("Inside setTimeout");
}, 0);
 
Promise.resolve().then(() => console.log("Inside Promise"));
 
console.log("End");

Output:

Start
End
Inside Promise
Inside setTimeout

Why This Order?

  1. console.log("Start") executes first (synchronous code).
  2. setTimeout is sent to the Web APIs (does not block execution).
  3. The Promise callback goes to the Microtask Queue.
  4. console.log("End") executes.
  5. The Microtask Queue (Promise callback) executes before the Callback Queue (setTimeout).
  6. Finally, setTimeout executes.

This happens because Microtasks (Promises, MutationObservers) have higher priority than Macrotasks (setTimeout, setInterval, etc.).

Visualizing the Event Loop

Imagine a to-do list:

  1. JavaScript executes all synchronous code first (Main Stack).
  2. Asynchronous tasks are sent to Web APIs (like setTimeout).
  3. Once the Call Stack is empty, the Event Loop checks the Microtask Queue (Promised tasks).
  4. After all Microtasks are processed, it picks up the Macrotasks (setTimeout, I/O).

Key Takeaways

✅ JavaScript executes synchronous code first.
✅ Promises (Microtasks) execute before setTimeout (Macrotasks).
✅ The Event Loop keeps the application responsive by handling async operations.

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