Node.js — Promises

Creating Promises

const myPromise = new Promise((resolve, reject) => {
    const success = true;
    if (success) {
        resolve('It worked!');
    } else {
        reject('Something went wrong');
    }
});

Consuming Promises

myPromise
    .then(result => console.log(result))
    .catch(error => console.error(error))
    .finally(() => console.log('Done'));

Chaining

fetchUser(1)
    .then(user => fetchPosts(user.id))
    .then(posts => renderPosts(posts))
    .catch(err => handleError(err));

Promise.all

const [users, posts] = await Promise.all([
    fetchUsers(),
    fetchPosts()
]);

Promise.allSettled

const results = await Promise.allSettled([
    fetch('/api/fast'),
    fetch('/api/slow')
]);

results.forEach(result => {
    if (result.status === 'fulfilled') {
        console.log(result.value);
    } else {
        console.error(result.reason);
    }
});

Promise.race

const result = await Promise.race([
    fetch('/api/primary'),
    fetch('/api/backup')
]);

Mini Practice

1. Create a promise
2. Chain promises
3. Use Promise.all
4. Handle rejection

Up Next

Continue with Async Await — modern async syntax.