Promise Cheatsheet

promise.js

// await must only be used if you want to execute something after promise to await the result

const createdUser = await createUser();
const user = await getUser();

// async must be used in function if you want to await something

(async () => {
  await foo();
})()

// if you return the promise, `await` is not needed

// execute promise and assign result to a variable

const user = await getUser();

// execute multiple promises in parallel and assign result to a variable

const createdUsers = await Promise.all([
  createUser(),
  createUser(),
]);

// … assign result to individual variables

const [ createdUserA, createdUserB ] = await Promise.all([
  createUser(),
  createUser(),
]);

// when using array like objects like Map, Set, Node you have to "borrow" some functions

const users = new Map()

Array.from(users).map(user => …)

// execute multiple promises in sequence and assign result to a variable
// Important: `Array.prototype.map` must return a value

const userIds = ['1', '2', '3'];

const users = await Promise.all(userIds.map(userId => getUserById(userId)));

// add check / condition within loop

const users = await Promise.all(userIds.map(userId => {
  if (!userId) {
    return null;
  }

  return getUserById(userId)
}));

// better: use `Array.prototype.filter()`

const users = await Promise.all(
  userIds
    .filter(userId => userId)
    .map(userId => getUserById(userId))
);

// execute different promise based on condition
// if must be wrapped in function

function getUserByIdOrSlug(idOrSlug) {
  if (!isUUID(idOrSlug)) {
    return getUserBySlug(idOrSlug);
  }

  return getUserById(idOrSlug);
}

const user = await getUserByIdOrSlug(idOrSlug);