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.

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2. Rewrite this callback code using Promises

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

Task: Convert to:

• a Promise
• async/await version

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

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

Hint: Use setTimeout + resolve.

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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.

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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.

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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.

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7. Create a function that retries a Promise 3 times

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

Hint: Loop + recursion + catch.

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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.

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9. Implement your own Promise.all()

Input:

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

Hint: Count resolved values + reject immediately.

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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.

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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.

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12. Why does this throw an error?

await Promise.resolve("hello");

Hint: Only allowed inside async functions.

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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.

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14. Explain and predict output

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

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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.

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