javascriptScope errorHard

Every callback reports index 3

Time: 0:00

BugFind the bug and fix the code

What it should do

collectIndexes(n) starts one delayed callback for each index and returns the indexes they report. Example: collectIndexes(3) → [0, 1, 2].

What happens instead

collectIndexes(3) gives [3, 3, 3].

function collectIndexes(n) {
    const out = [];
    const pending = [];
    for (var i = 0; i < n; i++) {
        pending.push(new Promise((resolve) => {
            setTimeout(() => { out.push(i); resolve(); }, 1);
        }));
    }
    return Promise.all(pending).then(() => out);
}

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 scope and closure bugs pattern

Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.

Learn the scope and closure bugs pattern Step through your own code

<iframe src="https://trybughunt.com/embed/cacd9a60-aefb-45dc-acd0-26d5d477557e" title="Debugging challenge: Every callback reports index 3" 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/cacd9a60-aefb-45dc-acd0-26d5d477557e">TryBugHunt</a></p>

function collectIndexes(n) { const out = []; const pending = []; for (var i = 0; i < n; i++) { pending.push(new Promise((resolve) => { setTimeout(() => { out.push(i); resolve(); }, 1); })); } return Promise.all(pending).then(() => out); }

Every callback reports index 3

What it should do

What happens instead

More scope and closure bugs to practise

Related errors

Every callback reports index 3

What it should do

What happens instead

More scope and closure bugs to practise

Related errors