// Sync callback
function greetings(callback) {
  callback();
}
greetings(() => { console.log('Hi'); });
moreWork(); // will run after console.log

// Async callback
const fs = require('fs');
fs.readFile('/file.md', function callback(err, data) { // fs.readFile is an async method provided by Node
  if (err) throw err;
  console.log(data);
});
moreWork(); // will run before console.log