Top Promise-Based Interview Coding Questions

I’ve included questions + small hints (not full solutions unless you want them).

1. What will be the output of this code?

console.log("A"); Promise.resolve().then(() => console.log("B")); console.log("C"); 

Hint: Think microtask queue vs sync.

Expected order: A, C, B.

2. Rewrite this callback code using Promises

function getData(cb) { setTimeout(() => cb("done"), 1000); } 

Task: Convert to:

• a Promise
• async/await version

3. Create a function delay(ms) that returns a Promise

delay(1000).then(() => console.log("1 second passed")); 

Hint: Use setTimeout + resolve.

4. Implement promisify

Convert a callback-style function:

function add(a, b, callback) { callback(null, a + b); } 

into:

const addPromise = promisify(add); addPromise(2, 3).then(console.log); 

Hint: Resolve on success, reject on error.

5. What is the output?

Promise.resolve(1) .then((x) => x + 1) .then((x) => { throw new Error("Oops"); }) .catch(() => 10) .then((x) => console.log(x)); 

Hint: .catch() returns a value → chain continues.

6. Run promises in parallel and return results in order

const tasks = [ () => Promise.resolve(10), () => Promise.resolve(20), () => Promise.resolve(30) ]; runAll(tasks).then(console.log); // expected: [10, 20, 30] 

Hint: Use Promise.all.

7. Create a function that retries a Promise 3 times

retry(() => fetch("/api"), 3) .then(console.log) .catch(console.error); 

Hint: Loop + recursion + catch.

8. Output prediction

async function test() { console.log(1); await Promise.resolve(); console.log(2); } console.log(3); test(); console.log(4); 

Hint: await → microtask.

Expected: 3, 1, 4, 2.

9. Implement your own Promise.all()

Input:

myPromiseAll([p1, p2, p3]).then(...); 

Hint: Count resolved values + reject immediately.

10. Difference between these two — output?

Promise.resolve().then(() => console.log("A")); Promise.resolve().then(console.log("B")); 

Hint: Second one logs B immediately because you're calling the function.

11. Chain Promises without using .then()

Given:

async function f1() { return 10; } async function f2() { return 20; } 

Call them one after another and sum the result.

12. Why does this throw an error?

await Promise.resolve("hello"); 

Hint: Only allowed inside async functions.

13. Turn an array of URLs into fetched data (Promise + map)

const urls = ["a.com", "b.com", "c.com"]; fetchAll(urls).then(console.log); 

Hint: Map to fetch → Promise.all.

14. Explain and predict output

Promise.reject("err") .then(() => console.log("then")) .catch(() => console.log("catch")) .then(() => console.log("after catch")); 

15. Create a timeout wrapper

timeout(fetch("/api"), 2000) .then(console.log) .catch(console.error); 

If Promise doesn’t resolve in 2 seconds → reject with timeout.