JavaScript is single-threaded, but it can still handle things like API calls, timers, and user actions without freezing the app.

What?

The event loop is a system that decides:

• What code runs now
• What runs later

It helps JavaScript handle asynchronous tasks smoothly.

Queues

Microtask Queue (high priority)

• Promise.then
• async/await

Macrotask Queue (low priority)

• setTimeout
• setInterval

How It Works

The event loop runs like this:

1. Run all synchronous code
2. Run all microtasks
3. Run one macrotask
4. Repeat

👉 Just remember:

Sync → Microtasks → Macrotasks

Example

console.log("A");

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

Promise.resolve()
  .then(() => { console.log("C");})
  .then(() => { console.log("D"); });

queueMicrotask(() => { console.log("E");});

(async function () {
  console.log("F");
  await null;
  console.log("G");
})();

console.log("H");

Output

F
H
C
E
D
G
B