[JS] One Mistake I Made with await in a Loop

[Lomanu4]

A while back, I built a utility script to process thousands of items with an asyncfunction. I thought slapping await inside a for() loop would do the trick. Spoiler: it didn’t!

That simple mistake taught me a big lesson about how await, for, map, and Promise.all work. As a back-end developer, I now use this trick to optimize APIs. In this post, I’ll break down what went wrong and how I fixed it with Promise.all.

Here’s the messy code I started with:


for (const item of items) {
await process(item);
}

At first, I thought I was a genius. Slapping await inside a for() Loop to process an array? Worked like a charm… until I hit 500+ items. My script crawled slower than a turtle stuck in traffic.

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The rookie mistake? I was running tasks one by one, like a single-threaded program. No parallelism, no speed. Oh my god.

The better approach was to use Promise.all


await Promise.all(arr.map(item => Process(item)));

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Boom! Execution time dropped from 5 seconds to 0.5 seconds. As a back-end dev, I now use this trick to turbocharge my APIs.

Lesson learned: If you don’t need strict order, avoid await inside loops. Always check if the task can be parallelized.

Here’s the full code to try it yourself:


async function Process(data) {
return new Promise(resolve => {
setTimeout(() => {
console.log(data);
resolve();
}, 500);
});
}

async function main() {
const start = process.hrtime.bigint();

const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];


for (const item of arr) {
await Process(item);
}

// parallelized
// await Promise.all(arr.map(item => Process(item)));

const end = process.hrtime.bigint();
const durationNanoseconds = end - start;
const durationSeconds = Number(durationNanoseconds) / 1_000_000_000;

console.log(`Sequential execution time: ${durationSeconds.toFixed(3)} seconds`);
}

main();

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Thanks for reading!

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