Callback functions in JavaScript

Synchronous operation

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

function createPost(post) {
    setTimeout(function () {
        posts.push(post);
    }, 2000);
}

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

createPost({title: "Post Three", body: "This is post three"});

getPosts();

Asynchronous operation

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

function createPost(post, callback) {
    setTimeout(function () {
        posts.push(post);
        callback();
    }, 2000);
}

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

createPost({title: "Post Three", body: "This is post three"}, getPosts);

References:
http://javascriptissexy.com/understand-javascript-callback-functions-and-use-them/
https://houssein.me/javascript/2016/05/10/asynchronous-javascript-callbacks.html
http://callbackhell.com/
https://developer.mozilla.org/en-US/docs/Glossary/Callback_function