returning non-Promise values from async functions and running them synchronously (or Promise.sync() idea)

# #!/JoePea (6 months ago)

I often find myself enjoying async functions until the time comes when I don't want to await anything, and I want the call stack to be sync (i.e. I don't want to defer if I don't have to).

But, async functions always return a Promise. So I find my self converting my async functions back into normal functions so that I can return non-Promise values, this way the caller can wait only if I return a promise, otherwise continue synchronously.

Here's an example function in one of my classes:

            initWebGL() {
                if (this.__glLoading) return false
                this.__glLoading = true

                // order of the if-else matters!
                if (this.__glUnloading) return
Promise.resolve().then(A.bind(this))
                else if (this.__glLoaded) return false

                A.call(this)

                function A() {
                    // ... load stuff (omitted for brevity) ...
                }

                return true
            }

then the caller (f.e. a subclass) only needs to wait when a promise is returned:

        initWebGL() {
            const superResult = super.initWebGL()
            if (superResult instanceof Promise) return
superResult.then(A.bind(this))
            if (!superResult) return false

            A.call(this)

            function A() {
                    // ... subclass loads stuff (omitted for brevity) ...
            }

            return true
        }

This is just one example, but in general I find many cases where I want to run synchronously most of the time, and only sometimes have to defer (whether by choice or not).

So I am losing out on the nice await syntax just because I can not do what I want to do with async/await.

What if async functions could have some way to continue synchronously, and return non-Promise values?

Or, what if we could have some sort of Promise API that would cause any callers to synchronously away, like in the following example, so that the change is not a breaking change, and async functions would still return Promises but the Promise could perhaps continue synchronously:

async function example() {
  return Promise.sync()
}

Then, in a caller, there would be no microtask deferral:

async function test() {
  await example()
  console.log('one')
  // implicit return is a Promise.sync() here.
}

function main() {
  test()
  console.log('two')
}

The output in this case would be:

"one"
"two"

Note that in that example, we are sure that two is logged after one because those async functions only ever use Promise.sync().

It would be good practice to use await regardless, because one may not know if the async function they are calling will return a non-Promise.sync value. So the main function is better written as

async function main() {
  await test()
  console.log('two')
}

In this case, everything still happens synchronously, but if the author of example() changes the implementation to return a non-sync Promise, then things will still run in the expected order. F.e.

async function example() {
  await fetch(...)
}

I keep finding scenarios where I want to avoid async unless I need async, but then I lose the convenient async/await syntax.

# Isiah Meadows (6 months ago)

You could move the async part into a helper function and call that from a sync function you instead expose. I've used this trick more than once, and although it is a bit of boilerplate, it works well enough.

# Jordan Harband (6 months ago)

Typically, APIs that are sometimes sync and sometimes async are called "z̲̗̼͙̥͚͛͑̏a̦̟̳͋̄̅ͬ̌͒͟ļ̟̉͌ͪ͌̃̚g͔͇̯̜ͬ̒́o̢̹ͧͥͪͬ" - unpredictable, hard to maintain, hard to understand. The general best practice is that a function should always be async, or always sync, but never the twain shall meet.

Relevant: blog.izs.me/post/59142742143/designing

# Isiah Meadows (6 months ago)

I agree that sometimes-sync is always a nightmare, and I've experienced this pain personally from a library API that once did this. (I did succeed in getting it to eventually change.) I'm willing to draw exception for things like APIs that wrap both sync and async iterators, but those literally provide two separate entry points in the form of Symbol.iterator vs Symbol.asyncIterator methods.


Isiah Meadows contact at isiahmeadows.com, www.isiahmeadows.com

# David Teller (6 months ago)

Unfortunately, turning async code (which often has components that execute in a different thread or process) back into in sync code is really, really hard.

What semantics would you give to this Promise.sync()? Can other code be executed by the main thread while you're waiting for your promise to be fulfilled?

  • If the answer is "no", you need to also block the entire DOM and CSS engines (both of which can call back into JavaScript) for this to work. Or, if you're in Node, all your I/O and all your ongoing callbacks. Needless to say, your users aren't going to be happy.

  • If the answer is "yes", you are breaking the run-to-completion guarantee of JavaScript. You may have events that are processed in the middle of apparently synchronous execution because there's a Promise.sync() hidden somewhere. That's going to break a lot of things.

For what it's worth, Firefox used to have an equivalent of Promise.sync() for use in privileged code (i.e. the UI and extensions). Experience shows that it caused insane amounts of bugs.

Cheers, David

P.S.: Yes, there are ways around this, if you're willing to write/use a CPS transpiler. I've written one years ago. The results were really powerful, but also really messy. I don't suggest heading in this direction.