Should you use await inside a for loop or Promise.all for independent tasks?

You have to pay five electricity bills online. If you pay one, wait for the receipt, then start the next, it takes five times as long. If you open five tabs and pay them together, it takes about as long as the slowest one. A loop with await is the first approach; Promise.all is the second.

const ids = [1, 2, 3, 4, 5];

// sequential: about 5 x 200 ms = 1000 ms
async function sequential() {
  const out = [];
  for (const id of ids) {
    out.push(await fetchStudent(id));
  }
  return out;
}

// concurrent: about 200 ms
async function concurrent() {
  return Promise.all(ids.map((id) => fetchStudent(id)));
}

Use the loop when:

• Each call needs the previous result.
• Order of side effects matters, such as numbered invoices.
• You must respect a rate limit or avoid overloading a database.

For thousands of items, even Promise.all is risky, since it starts everything at once. Process in batches of, say, 10 to 50 at a time, or use a limiter library such as p-limit.

Also, array.forEach(async ...) does not wait for anything. Use for...of or Promise.all(map).

• Using forEach with async callbacks and expecting it to finish before the next line.
• Firing ten thousand promises at once.