What is Promise.all and when to use it

Short answer

Promise.all takes an iterable of Promises and returns a single Promise that resolves when all input Promises resolve, or rejects immediately if any reject.

How it works

• Resolves to an array of results in the same order as the input.
• Rejects with the first rejection reason, ignoring the rest.

Example

const [users, posts] = await Promise.all([
  fetch("/users").then(r => r.json()),
  fetch("/posts").then(r => r.json())
]);

When to use each

Tips

• Wrap independent async calls in Promise.all to run them in parallel.
• Use allSettled when you need to handle partial failures gracefully.

Common issues

• Passing non-Promise values works but is unnecessary; wrap them if unsure.
• A single rejection in Promise.all causes the whole operation to fail.