Simple implementation of taking a callback-style function that accepts an `(err, result)` callback as its last argument and return that function promisified.

promisify.js

// dummy callback func; randomly calls success or failure callback after 1 sec
const myFunc = (a, b, callback) => {
  setTimeout(() =>
    Math.random() > 0.5 ? callback(null, 'success!') : callback('fail :('),
  1000)
}

// test the dummy func
myFunc('foo', 'bar', (err, res) => {
  if (err) {
    console.log(err)
  } else {
    console.log(res)
  }
})


// take a function that accepts a "(err, result)"-style callback as its last
// argument and return a function that calls the passed function, but returns
// a promise that will resolve or reject
// use a `function` function so the `arguments` array is set to the proper
// value below
const promisify = (funcWithCallback) => function() {
  return new Promise((resolve, reject) =>
    funcWithCallback(...arguments, (err, res) =>
      err ? reject(err) : resolve(res)
    )
  )
}

// "promisify" the callback-style function
const promisifiedFunc = promisify(myFunc)

// test it out!
try {
  const res = await promisifiedFunc('foo', 'bar')
  console.log(res)
} catch (err) {
  console.log(err)
}