Converting a callback to a promise

Sometimes you want your synchronous function to run asynchronously. Perhaps you want to run multiple functions asynchronously using something like Promise.allSettled or Promise.all.

I have a number of setup functions that dont depend on each other in an application and I was curious how hard it would be to convert the setup functions to async functions without touching their internal code. (Some functions come from libraries)

The TLDR is that yes, I managed to do it.

function asPromise (callback, ...args) {
  return new Promise((resolve, reject) => {
    try {
      resolve(callback(...args))
    } catch(e) {
      reject(e)
    }
  })
}

Now for some examples:

function greet (greeting, name) { return "${greeting}, {name}" }
await asPromise(greet, "hi", "konnor")
// => "hi, konnor"

Now what if we pass an object?

function greet ({greeting, name}) { return "${greeting}, {name}" }
await asPromise(greet, {greeting: "hi", name: "konnor"})
// => "hi, konnor"

And finally, what about an array?

function greet (ary) {
  return `${ary[0]}, ${ary[1]}`
}

await asPromise(greet, ["hi", "konnor"])
// => "hi, konnor"

Are there edge cases? Probably. Mostly around this

if your function calls rely on this make sure to bind within the Promise like so:

await asPromise(myFunction.bind(myThis), "arg1")

And that’s all for today! Short and sweet.