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Top 25 JavaScript Event Loop Interview Questions & Answers 2026

Master JavaScript event loop interview questions with call stack, microtasks, macrotasks, Promise order, async/await execution, and real output prediction examples.

The event loop is always asked in senior JavaScript interviews. Understanding it separates juniors from seniors. Master this, and async JavaScript will never confuse you again.

Tips for Interview 💡

Always say: *"The event loop picks from the microtask queue before the macrotask queue."*
Promise.resolve().then(...) = microtask (runs before setTimeout)
setTimeout(fn, 0) = macrotask (runs after all Promises)
async/await is just syntactic sugar over Promises — awaited code is microtask.
The call stack must be empty before the event loop picks new tasks.

Q1: The Classic Order Question

js example

console.log("1");

setTimeout(function () {
  console.log("2");
}, 0);

Promise.resolve().then(function () {
  console.log("3");
});

console.log("4");

Output

Step-by-step:

console.log("1") → sync → prints 1
setTimeout → registered → goes to macrotask queue
Promise.resolve().then(...) → microtask scheduled
console.log("4") → sync → prints 4
Stack empty → check microtask queue → prints 3
Microtask queue empty → pick from macrotask → prints 2

Q2: Multiple Promises Order

Example

console.log("start"); Promise.resolve("A").then(val => { console.log(val); return "B"; }).then(val => { console.log(val); }); Promise.resolve("C").then(val => { console.log(val); }); console.log("end");

Output

start
end
A
C
B

Why? Both .then("A") and .then("C") are scheduled first. "B" is scheduled only after "A" resolves.

Q3: async/await Execution Order

js example

async function foo() {
  console.log("foo start");
  const result = await Promise.resolve("awaited");
  console.log(result);
  console.log("foo end");
}

console.log("before");
foo();
console.log("after");

Output

before
foo start
after
awaited
foo end

Why? await suspends foo at that line, returns control to the caller (console.log("after")), then resumes as a microtask.

Q4: Nested Promises

Example

Promise.resolve() .then(() => { console.log("1"); return Promise.resolve("nested"); }) .then(val => { console.log("2:", val); }); Promise.resolve().then(() => console.log("3"));

Output

1
3
2: nested

Why? Returning a Promise from .then adds two extra microtask hops. The "3" microtask sneaks in between "1" and "2".

Q5: setTimeout Inside Promise

Example

console.log("sync 1"); setTimeout(() => console.log("timeout"), 0); Promise.resolve() .then(() => { console.log("promise 1"); setTimeout(() => console.log("timeout inside promise"), 0); }) .then(() => console.log("promise 2")); console.log("sync 2");

Output

sync 1
sync 2
promise 1
promise 2
timeout
timeout inside promise

Q6: async/await vs Promise.then Order

Example

async function asyncFunc() { console.log("async A"); await null; console.log("async B"); } Promise.resolve().then(() => console.log("then C")); asyncFunc(); Promise.resolve().then(() => console.log("then D")); console.log("sync E");

Output

async A
sync E
async B
then C
then D

Wait, let's be precise:

Output

async A
sync E
then C
async B
then D

Note: await null schedules one microtask tick. then C was queued before asyncFunc was called in terms of the microtask order.

Q7: Promise.all Execution Order

Example

console.log("start"); Promise.all([ new Promise(resolve => { console.log("P1 created"); setTimeout(() => resolve("P1"), 100); }), new Promise(resolve => { console.log("P2 created"); setTimeout(() => resolve("P2"), 50); }) ]).then(results => console.log("All:", results)); console.log("end");

Output

start
P1 created
P2 created
end
All: ["P1", "P2"]

Note: Promise.all waits for ALL promises. Results are in input order, not resolution order.

Q8: Error in async/await

js example

async function fetchUser() {
  try {
    const result = await Promise.reject(new Error("Not found"));
    console.log(result);
  } catch (err) {
    console.log("Caught:", err.message);
  }
  console.log("After try/catch");
}

fetchUser();
console.log("Sync after call");

Output

Sync after call
Caught: Not found
After try/catch

Q9: Microtask Starvation

Example

function recursiveMicrotask(n) { if (n === 0) { console.log("Microtasks done!"); return; } Promise.resolve().then(() => recursiveMicrotask(n - 1)); } recursiveMicrotask(3); setTimeout(() => console.log("setTimeout"), 0);

Output

Microtasks done!
setTimeout

Why? All microtasks (even recursively queued ones) are drained before any macrotask runs. setTimeout only runs after all microtasks complete.

Q10: queueMicrotask vs Promise

Example

console.log("1"); queueMicrotask(() => console.log("2 - queueMicrotask")); Promise.resolve().then(() => console.log("3 - Promise")); console.log("4");

Output

1
4
2 - queueMicrotask
3 - Promise

Both are microtasks — they run in the order they were queued.

Q11: setInterval and Clearing

Example

let count = 0; const id = setInterval(() => { count++; console.log("Tick:", count); if (count === 3) { clearInterval(id); console.log("Cleared!"); } }, 100);

Output

Tick: 1
Tick: 2
Tick: 3
Cleared!

Q12: Async Iterator / for-await

Example

async function run() { const promises = [ Promise.resolve(1), Promise.resolve(2), Promise.resolve(3) ]; for await (const val of promises) { console.log(val); } } run();

Output

1
2
3

Q13: Promise Constructor Executor

Example

console.log("A"); new Promise((resolve) => { console.log("B"); // Executor runs SYNCHRONOUSLY resolve("C"); }).then(val => console.log(val)); console.log("D");

Output

A
B
D
C

Key: The Promise executor function runs synchronously. Only .then callbacks are async (microtasks).

Q14: Multiple awaits

Example

async function multi() { console.log("1"); await Promise.resolve(); console.log("2"); await Promise.resolve(); console.log("3"); } multi(); console.log("4"); Promise.resolve().then(() => console.log("5"));

Output

Why? After each await, control returns to the event loop. Other microtasks (like "5") can run between awaits.

Q15: Event Loop + DOM Events

Example

// Conceptual — how it works in browsers button.addEventListener("click", () => { console.log("click handler"); Promise.resolve().then(() => console.log("microtask after click")); setTimeout(() => console.log("timeout after click"), 0); }); // When clicked: // click handler // microtask after click // timeout after click

Common Traps 🪤

Pattern	Queue Type	When Runs
`Promise.resolve().then(...)`	Microtask	After current sync, before setTimeout
`setTimeout(fn, 0)`	Macrotask	After ALL microtasks
`queueMicrotask(fn)`	Microtask	After current sync, before setTimeout
`async/await` continuation	Microtask	After current sync
`setInterval(fn, n)`	Macrotask	Repeatedly, after delay
Promise executor `new Promise(fn)`	Synchronous	Right now

Interview Questions Quick Reference

Q	Answer
What is the event loop?	Mechanism that moves tasks from queues to call stack when stack is empty
Microtask vs Macrotask?	Microtasks (Promises, queueMicrotask) run before macrotasks (setTimeout)
Why `setTimeout(fn, 0)` isn't instant?	It's a macrotask — runs after all microtasks
Is Promise executor sync or async?	Synchronous — runs immediately
What drains first?	All microtasks → then ONE macrotask → all microtasks → ...

Practice Self-Test

Example

// Predict the output: console.log("1"); setTimeout(() => console.log("2"), 0); Promise.resolve() .then(() => { console.log("3"); return Promise.resolve(); }) .then(() => console.log("4")); queueMicrotask(() => console.log("5")); console.log("6");

Output

Summary

text example

Event Loop Order:
1. Execute all synchronous code
2. Drain entire microtask queue (Promises, queueMicrotask)
3. Run ONE macrotask (setTimeout, setInterval, I/O)
4. Go to step 2

Master this mental model and any output question becomes predictable!

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