Rethinking Async Loops in JavaScript

Summary

Using await in loops seems intuitive until your code silently stalls or runs slower than expected. If you’ve ever wondered why your API calls run one-by-one instead of all at once, or why map() and await don’t mix the way you’d expect, grab a chair. Let’s chat. The problem: awaiting in a for loop Suppose you’re fetching a list of users one-by-one: const users = [1, 2, 3]; for (const id of users) { const user = await fetchUser(id); console.log(user); } This works, but it runs sequentially: fetchUser(2) doesn’t start until fetchUser(1) finishes. That’s fine if order matters, but it’s inefficient for independent network calls. Don’t await inside map() unless you mean to A common point of confusion is using await inside map() without handling the resulting promises: const users = [1, 2, 3]; const results = users.map(async id => { const user = await fetchUser(id); return user; }); console.log(results); // [Promise, Promise, Promise] – NOT actual user data This works in terms of syntax and behavior (it returns an array of promises), but not in the way many expect. It doesn’t wait for the promises to resolve. To run the calls in parallel and get the final results: const results = await Promise.all(users.map(id => fetchUser(id))); Now all the requests run in parallel, and results contains the actual fetched users. Promise.all() fails fast, even if just one call breaks When using Promise.all(), a single rejection causes the entire operation to fail: const results = await Promise.all( users.map(id => fetchUser(id)) // fetchUser(2) might throw ); If fetchUser(2) throws an error (e.g., 404 or network error), the entire Promise.all call will reject, and none of the results will be returned (including successful ones). ⚠️ Note: Promise.all() rejects on the first error, discarding other results. Remaining promises still run, but only the first rejection is reported unless you handle each one individually. Safer alternatives Use Promise.allSettled() const results = await Promise.al...