What do you understand about event loop?

The event loop coordinates synchronous and asynchronous code.

The key parts:

Call Stack

Microtask Queue

Task / Macrotask Queue

Example:

console.log("A");

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

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

console.log("D");

Output:

A
D
C
B

Why?

First, all the synchronous code runs:

A
D

The Promise callback goes to the microtask queue.

The setTimeout callback goes to the task/macrotask queue.

Once the call stack is empty, the event loop runs the microtasks first.

That's why:

Promise

comes before:

setTimeout

To be precise:
A Promise doesn't run .then() immediately either. Its callback also waits, just in a higher-priority queue.