javascriptAsync race conditionHard

Cached loader runs 3 times instead of once

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What it should do

countLoads(n) calls a cached loader n times at once and returns how many times the slow load really ran. It should always be 1. Example: await countLoads(3) → 1.

What happens instead

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

async function countLoads(callCount) {
    let loads = 0;
    let cache = null;

    const getConfig = async () => {
        if (!cache) {
            loads = loads + 1;
            await new Promise((r) => setTimeout(r, 5));
            cache = { ready: true };
        }
        return cache;
    };

    const calls = [];
    for (let i = 0; i < callCount; i++) calls.push(getConfig());
    await Promise.all(calls);
    return loads;
}

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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 countLoads(callCount) { let loads = 0; let cache = null; const getConfig = async () => { if (!cache) { loads = loads + 1; await new Promise((r) => setTimeout(r, 5)); cache = { ready: true }; } return cache; }; const calls = []; for (let i = 0; i < callCount; i++) calls.push(getConfig()); await Promise.all(calls); return loads; }

Cached loader runs 3 times instead of once

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Cached loader runs 3 times instead of once

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