codeSamples/callbacks.js

'use strict';
// Run and see random order
// Use global counter to detect finish (bad practice)

const callbackCounter = (count, callback) => {
  let counter = 0;
  const done = () => { if (++counter === count) callback(); };
  return done;
};
  
// Emulate asynchronous calls
const wrapAsync = (fn) => (...args) => setTimeout(
  () => fn(...args), Math.floor(Math.random() * 1000)
);
  
// Asynchronous functions 
const readConfig = wrapAsync((name, callback) => {
  console.log('(1) config loaded');
  callback(null, { name });
});

const doQuery = wrapAsync((statement, callback) => {
  console.log('(2) SQL query executed: ' + statement);
  callback(null, [{ name: 'Scorpio' }, { name: 'Spider' }]);
});
  
const httpGet = wrapAsync((url, callback) => {
  console.log('(3) Page retrieved: ' + url);
  callback(null, '<html>Some archaic web here</html>');
});
  
const readFile = wrapAsync((path, callback) => {
  console.log('(4) ' + path + ' file loaded');
  callback(null, 'file content');
});
  
console.log('start'); 
const callback = callbackCounter(4, () => { console.log('All done!'); }); 
  readConfig('myConfig', callback);
  doQuery('select * from companies', callback);
  httpGet('http://skj.com', callback);
  readFile('README.md', callback);  
console.log('end');
