async and await

An async function always returns a promise. The await keyword pauses that async function until a promise settles, then produces its fulfilled value or throws its rejection. It does not block the entire JavaScript runtime while waiting.

async function loadProfile() {
  const profile = await Promise.resolve({ name: "Asha" });
  return profile;
}

loadProfile().then(console.log);

Inside an async function, try...catch can handle rejected operations in a sequential style:

async function loadData() {
  try {
    const result = await Promise.resolve("Loaded");
    return result;
  } catch (error) {
    console.error("Could not load data", error);
    throw error;
  }
}

Catch errors where you can recover or add context. If you cannot recover, rethrowing may let the caller decide what to do. Avoid catching an error and returning a fake success value without making that choice explicit.

Sequential versus parallel#

Awaiting independent tasks one after another makes their total duration add up. When tasks do not depend on each other, start them together and await Promise.all:

const [user, settings] = await Promise.all([
  fetchUser(),
  fetchSettings(),
]);

Practice#

Convert a promise chain into an async function. Add a try...catch block and explain whether two independent tasks should run sequentially or concurrently.