JavaScript Developer Free Lesson: The Event Loop

AP3: The event loop, tasks and microtasks

You’ll be able to: predict the order of logs from sync code, promise callbacks and timers.

Run order

Run to completion

JavaScript runs one job at a time. When the stack is empty, every queued microtask runs, then the next task. A setTimeout with 0 ms still waits for all of that.

1console.log('A');
2setTimeout(() => console.log('B'), 0);
3Promise.resolve().then(() => console.log('C'));
4queueMicrotask(() => console.log('D'));
5console.log('E');
6// A, E, C, D, B

Practice questions

Ten exam-style questions on this lesson. Answer each one before you open it.

Question 1. console.log(1); setTimeout(() => console.log(2), 0); Promise.resolve().then(() => console.log(3)); console.log(4); What’s the order?

• A. 1, 2, 3, 4
• B. 1, 4, 3, 2
• C. 1, 4, 2, 3
• D. 1, 3, 4, 2, because a resolved then runs at once

Question 2. Which callbacks run as microtasks? (Choose 2.)

• A. Promise then callbacks
• B. queueMicrotask callbacks
• C. setTimeout callbacks
• D. click handlers that the user triggered earlier

Question 3. A 3-second synchronous loop runs right after setTimeout(fn, 0). When does fn run?

• A. After 0 ms, interrupting the loop
• B. Never
• C. In parallel, on another thread
• D. After the loop finishes

Question 4. What does run-to-completion mean?

• A. Every promise must fulfill
• B. Async functions finish before the page can paint again
• C. Each job finishes before another starts
• D. Timers always run on time

Question 5. setTimeout(fn, 1000) is called. What does the delay guarantee?

• A. fn runs no sooner than about 1 second
• B. fn runs before any promise callback queued after it
• C. fn runs within 1 second
• D. fn runs exactly at 1 second

Question 6. async function f() { console.log(‘a’); await null; console.log(‘b’); } f(); console.log(‘c’); What’s the order?

• A. a, b, c
• B. c, a, b
• C. a, b, c, since await null doesn’t pause
• D. a, c, b

Question 7. Which statements about the event loop are true? (Choose 2.)

• A. Promise callbacks run in a new thread
• B. setTimeout 0 runs before microtasks
• C. Microtasks run before the next task
• D. A long sync loop blocks rendering

Question 8. A click handler’s promise resolves, and a 0 ms timer was also queued earlier. What runs next after the handler finishes?

• A. The timer
• B. The promise’s then callback
• C. The next click, if the user clicked again
• D. A repaint, always

Question 9. Which API queues a microtask directly?

• A. setInterval(fn, 0) with a cleared id
• B. requestAnimationFrame()
• C. queueMicrotask()
• D. setTimeout(fn, 0)

Question 10. Promise.resolve().then(() => console.log(‘x’)); console.log(‘y’); What’s logged first?

• A. y
• B. Both at once
• C. x
• D. Neither; the then callback needs await

Sources

• MDN: JavaScript execution model
• MDN: Using microtasks in JavaScript with queueMicrotask()
• MDN: Window: queueMicrotask() method
• MDN: Window: setTimeout() method
• MDN: Using promises

Checked against MDN Web Docs, Node.js, npm, Jest, Chrome DevTools docs and Trailhead on Oct 23, 2026. Questions follow the current JavaScript Developer exam guide.