callback.js

const posts = [
    {title: 'Post One', body: 'This is a post 1'},
    {title: 'Post Two', body: 'This is a post 2'},
]

/**
 * Print in the DOM all posts, each one wait 1 second to be posted.
 */
function getPosts() {
    setTimeout(() => {
        let output = '';
        posts.forEach((post, index) => {
            output += `<li>${post.title}</li>`;
        });
        document.body.innerHTML = output;
    }, 1000);
}

/**
 * 
 * @param { JSON Object } post 
 * @param { Function } callback 
 * The post will be added to posts and then the callback will be called 
 * The function usual pass is the getPost to refresh the posts in DOM. 
 * 
 */
function createPost(post, callback) {
    setTimeout(() => {
        let newPost = {title: post.title, body: post.body };
        posts.push(newPost);
        callback();
    }, 2000);
}

// Execution
getPosts();
createPost({title: 'oi', body: 'ahahah', lixo: 'Will I overflow?'}, getPosts);
createPost({title: 'de novo', body: 'hihihi', lixo: 'Will I overflow?'}, getPosts);