javascriptAsync race conditionHard

Doubling with forEach returns an empty list

Time: 0:00

BugFind the bug and fix the code

What it should do

doubleAll(nums) doubles every number with fetchDouble, which takes a moment, and returns the results. Example: await doubleAll([1, 2, 3]) → [2, 4, 6].

What happens instead

await doubleAll([1, 2, 3]) gives [].

function fetchDouble(n) {
    return new Promise((resolve) => setTimeout(() => resolve(n * 2), 5));
}

async function doubleAll(nums) {
    const results = [];
    nums.forEach(async (n) => {
        results.push(await fetchDouble(n));
    });
    return results;
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

⏱️0:00

Hints

🏆 0 hints = Flawless Solve

Think & Test Cooldown

2:00 remaining

Take a moment to read the bug symptom and trace the code with Run & Check before unlocking Hint 1.

Hint 2

Unlocks after Hint 1

Hint 3

Unlocks after Hint 2

This bug belongs to the async and race conditions pattern

A missing await, results in the wrong order, and updates that overwrite each other under load.

Learn the async and race conditions pattern Step through your own code

<iframe src="https://trybughunt.com/embed/4d804a81-0561-4249-b0fb-94a48bf69c4a" title="Debugging challenge: Doubling with forEach returns an empty list" width="100%" height="580" loading="lazy" style="border:1px solid #d4d4d8;border-radius:12px;max-width:760px"></iframe> <p style="font-size:14px">Debugging challenge from <a href="https://trybughunt.com/challenges/4d804a81-0561-4249-b0fb-94a48bf69c4a">TryBugHunt</a></p>

function fetchDouble(n) { return new Promise((resolve) => setTimeout(() => resolve(n * 2), 5)); } async function doubleAll(nums) { const results = []; nums.forEach(async (n) => { results.push(await fetchDouble(n)); }); return results; }

Doubling with forEach returns an empty list

What it should do

What happens instead

More async and race conditions to practise

Related errors

Doubling with forEach returns an empty list

What it should do

What happens instead

More async and race conditions to practise

Related errors