javascriptAsync race conditionHard

3 people book the last seat

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BugFind the bug and fix the code

What it should do

bookSeats(n) sends n booking requests at the same time for 1 remaining seat, and returns how many bookings succeeded. It should never be more than 1. Example: await bookSeats(3) → 1.

What happens instead

await bookSeats(3) gives 3. await bookSeats(1) correctly gives 1.

async function bookSeats(requests) {
    let seatsLeft = 1;
    let booked = 0;

    const attempts = [];
    for (let i = 0; i < requests; i++) {
        attempts.push(
            (async () => {
                if (seatsLeft > 0) {
                    await new Promise((r) => setTimeout(r, 5));
                    seatsLeft = seatsLeft - 1;
                    booked = booked + 1;
                }
            })()
        );
    }
    await Promise.all(attempts);
    return booked;
}

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

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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.

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async function bookSeats(requests) { let seatsLeft = 1; let booked = 0; const attempts = []; for (let i = 0; i < requests; i++) { attempts.push( (async () => { if (seatsLeft > 0) { await new Promise((r) => setTimeout(r, 5)); seatsLeft = seatsLeft - 1; booked = booked + 1; } })() ); } await Promise.all(attempts); return booked; }

3 people book the last seat

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3 people book the last seat

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