An easy way to do async APIs in JavaScript that support both promises *and* callbacks!

resolvePromise.js

// Here is a function that I use all the time when creating public
// async APIs in JavaScript:
const resolvePromise = (promise, callback) => {
  if (callback)
    promise.then(value => callback(null, value), callback)

  return promise
}

// Sometimes I like to use callbacks, but other times a promise is
// easier to use. And sometimes it's hard to know up front which style
// you'll need later on. resolvePromise is an easy way to support both.

// Here, we wrap doSomethingAsync (could be any promise-only API, e.g.
// window.fetch) to give consumers the flexibility to use a trailing
// callback arg if they prefer callbacks to promises.
const somethingAsync = (a, b, c, callback) =>
  resolvePromise(
    doSomethingAsync(a, b, c), // Can be any promise-only API
    callback
  )
  
// Now, you can use either style:

somethingAsync(a, b, c).then(value => {
  // Promises work!
}, error => {
  // Use a separate function for handling errors.
})

somethingAsync(a, b, c, (error, value) => {
  // Callbacks too! Use one function to handle both success and error.
})