Promisify error-first callback based functions.

promisify.js

function promisify(fn) {
  return (...args) => {
    return new Promise((resolve, reject) => {
      fn(...args, (error, result) => {
        if (error) {
          return reject(error);
        }

        return resolve(result);
      });
    });
  }
}

// Examples
const fs = require('fs');

const readFile = promisify(fs.readFile);

readFile('test.txt')
  .then(console.log)
  .catch(console.error);