an example how javascript callback works

callback.js

console.log('Before');

getUser(1, displayUser);

console.log("After");

function getUser(id, callback){
  setTimeout(() => {
    console.log("Reading a user from database!");
    callback({ id: id, name: "salmanx" })
  }, 2000)  
}

function getRepositories(username, callback){
  setTimeout(() => {
    console.log("Calling github api");
    callback(["repo1", "repo2", "repo3"]);
  }, 2000)
}

function getCommits(repo, callback){
  setTimeout(() => {
    callback(["commit1", "commit2"]);
  }, 2000)
}

function displayCommits(commits){
  console.log(commits);
}

function displayRepos(repos){
  getCommits(repos[0], displayCommits);
}

function displayUser(user){
  getRepositories(user.name, displayRepos)
}