Some demo files I am using in a Medium article I write on NodeJS's util.promisify()

example-error

node promisify.js
Waiting ...
Waiting ...
Waiting ...
Waiting ...
Waiting ...
Waiting ...
Waiting ...
Waiting ...
[Error]: Could not wait any longer!

example-success

node promisify.js
Waiting ...
Waiting ...
Waiting ...
Waiting ...
Waiting ...
Congratulations, you have finished waiting.

promisify.js

const util = require('util');

// Here we use util.promisify to convert the function to a promise
const waitAsync = util.promisify(wait);

//And here we use the promisified function. Cool huh?
waitAsync(1000)
  .then(data => console.log(data))
  .catch(err => console.error(`[Error]: ${err}`));

wait-callback.js

// Calling wait and passing a callback
wait(1000, (err, data) => {
  // Did the function return an error?
  if (err) throw new Error(err);
  // Output the data
  console.log(data);
});

wait.js

// This function waits for an unknown amount of time
const wait = (delay, callback) => {
  // setInterval returns an ID. We need this to stop the timer
  const id = setInterval(() => {
    // Generate a random number between 0 and 1
    const rand = Math.random();

    if (rand > 0.95) {
      // Call the call back function. Note the first parameter is an error
      callback(null, 'Congratulations, you have finished waiting.');
      // Stop the timer
      clearInterval(id);
    } else if (rand < 0.01) {
      // Call the callback function. Note we're setting an error now
      callback('Could not wait any longer!', null);
      // Stop the timer
      clearInterval(id);
    } else {
      // Print to stdouts
      console.log('Waiting ...');
    }
  }, Number(delay));
};