Promise

HTML

<ul id="posts"></ul>

JavaScript

const posts = [
  { title: 'Post One', body: 'This is post one' },
  { title: 'Post Two', body: 'This is post two' }
];

function createPost(post) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      posts.push(post);
      const error = false;
      if (!error) {
        resolve();
      } else {
        reject('Error: Something went wrong');
      }
    }, 2000);
  });
}

function getPosts() {
  setTimeout(() => {
    let output = '';
    posts.forEach((post, index) => {
      output += `<li>${post.title}</li>`;
    });
    document.getElementById('posts').innerHTML = output;
  }, 1000);
}

createPost({ title: 'Post Three', body: 'This is post three' })
  .then(getPosts)
  .catch(err => console.log(err));

Code from Brad Traversy's tutorial:
Async JS Crash Course - Callbacks, Promises, Async Await
https://www.youtube.com/watch?v=PoRJizFvM7s

Demo

Promise using .catch

new Promise((resolve, reject) => {
    console.log('Initial');
    resolve();
})
.then(() => {
    throw new Error('Something failed');
    console.log('Do this');
})
.catch(() => {
    console.error('Do that');
})
.then(() => {
    console.log('Do this, no matter what happened before');
});

MDN Web Docs
Using Promises
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises