javascriptAsync race conditionMedium

Total shows "[object Promise]" instead of 6

Time: 0:00

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What it should do

sumDelayed(nums) adds up numbers that each arrive after a short delay. Example: await sumDelayed([1, 2, 3]) → 6.

What happens instead

await sumDelayed([1, 2, 3]) gives "0[object Promise][object Promise][object Promise]".

function delay(n) {
    return new Promise((resolve) => setTimeout(() => resolve(n), 1));
}

async function sumDelayed(nums) {
    let total = 0;
    for (const n of nums) {
        total += delay(n);
    }
    return total;
}

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.

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

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function delay(n) { return new Promise((resolve) => setTimeout(() => resolve(n), 1)); } async function sumDelayed(nums) { let total = 0; for (const n of nums) { total += delay(n); } return total; }

Total shows "[object Promise]" instead of 6

What it should do

What happens instead

More async and race conditions to practise

Seen this error in your own code?

Total shows "[object Promise]" instead of 6

What it should do

What happens instead

More async and race conditions to practise

Seen this error in your own code?