Using "await" inside non-async function

You can call an async function from within a non-async function via an Immediately Invoked Function Expression (IIFE):

(async () => await updateCacheForKey([key]))();

And as applied to your example:

function syncFunc(key) {
   if (!(key in cache)) {
      (async () => await updateCacheForKey([key]))();
   }
}

async function updateCacheForKey(keys) {
   // updates cache for given keys
   ...
}

My problem is I'd like the synchronous function to be able to wait for a value from the async one...

They can't, because:

1. JavaScript works on the basis of a "job queue" processed by a thread, where jobs have run-to-completion semantics, and

2. JavaScript doesn't really have asynchronous functions — even async functions are, under the covers, synchronous functions that return promises (details below)

The job queue (event loop) is conceptually quite simple: When something needs to be done (the initial execution of a script, an event handler callback, etc.), that work is put in the job queue. The thread servicing that job queue picks up the next pending job, runs it to completion, and then goes back for the next one. (It's more complicated than that, of course, but that's sufficient for our purposes.) So when a function gets called, it's called as part of the processing of a job, and jobs are always processed to completion before the next job can run.

Running to completion means that if the job called a function, that function has to return before the job is done. Jobs don't get suspended in the middle while the thread runs off to do something else. This makes code dramatically simpler to write correctly and reason about than if jobs could get suspended in the middle while something else happens. (Again it's more complicated than that, but again that's sufficient for our purposes here.)

So far so good. What's this about not really having asynchronous functions?!

Although we talk about "synchronous" vs. "asynchronous" functions, and even have an async keyword we can apply to functions, a function call is always synchronous in JavaScript. An async function is a function that synchronously returns a promise that the function's logic fulfills or rejects later, queuing callbacks the environment will call later.

Let's assume updateCacheForKey looks something like this:

async function updateCacheForKey(key) {
    const value = await fetch(/*...*/);
    cache[key] = value;
    return value;
}

What that's really doing, under the covers, is (very roughly, not literally) this:

function updateCacheForKey(key) {
    return fetch(/*...*/).then(result => {
        const value = result;
        cache[key] = value;
        return value;
    });
}

(I go into more detail on this in Chapter 9 of my recent book, JavaScript: The New Toys.)

It asks the browser to start the process of fetching the data, and registers a callback with it (via then) for the browser to call when the data comes back, and then it exits, returning the promise from then. The data isn't fetched yet, but updateCacheForKey is done. It has returned. It did its work synchronously.

Later, when the fetch completes, the browser queues a job to call that promise callback; when that job is picked up from the queue, the callback gets called, and its return value is used to resolve the promise then returned.

My question is why absolutely prevent this use case in the first place?

Let's see what that would look like:

1. The thread picks up a job and that job involves calling syncFunc, which calls updateCacheForKey. updateCacheForKey asks the browser to fetch the resource and returns its promise. Through the magic of this non-async await, we synchronously wait for that promise to be resolved, holding up the job.

2. At some point, the browser's network code finishes retrieving the resource and queues a job to call the promise callback we registered in updateCacheForKey.

3. Nothing happens, ever again. :-)

...because jobs have run-to-completion semantics, and the thread isn't allowed to pick up the next job until it completes the previous one. The thread isn't allowed to suspend the job that called syncFunc in the middle so it can go process the job that would resolve the promise.

That seems arbitrary, but again, the reason for it is that it makes it dramatically easier to write correct code and reason about what the code is doing.

But it does mean that a "synchronous" function can't wait for an "asynchronous" function to complete.

There's a lot of hand-waving of details and such above. If you want to get into the nitty-gritty of it, you can delve into the spec. Pack lots of provisions and warm clothes, you'll be some time. :-)

• Jobs and Job Queues
• Execution Contexts
• Realms and Agents