JS async/await - why does await need async?

15
votes

Why does using await need its outer function to be declared async?

For example, why does this mongoose statement need the function it's in to return a promise?

async function middleware(hostname, done) {
  try {
    let team = await Teams.findOne({ hostnames: hostname.toLowerCase() }).exec();
    done(null, team);
  } catch (err) { done(err); }
}

I see the runtime/transpiler resolving the Teams promise to it's value and async signaling it "throws" rejected promises.

But try/catch "catches" those rejected promises, so why are async and await so tightly coupled?

3
javascriptasynchronousasync-await
I don't get what you mean by "But try/catch "catches" those rejected promises". How is that relevant for the async keyword? – Bergi
Possible duplicate of Why is it necessary to have the async keyword? – Bergi
"why does this mongoose statement need the function it's in to return a promise?" - how else would it be able to wait for the asynchronous result of the mongoose promise? – Bergi
Here's a reason for the language design: stackoverflow.com/a/41744179/1483977 – Michael Cole

3 Answers

8
votes

I'm not privy to the JavaScript language design discussions, but I assume it's for the same reasons that the C# language requires async (also see my blog).

Namely:

  1. Backwards compatibility. If await was suddenly a new keyword everywhere, then any existing code using await as a variable name would break. Since await is a contextual keyword (activated by async), only code that intends to use await as a keyword will have await be a keyword.
  2. Easier to parse. async makes asynchronous code easier to parse for transpilers, browsers, tools, and humans.
6
votes

Copied from https://stackoverflow.com/a/41744179/1483977 by @phaux:

These answers all give valid arguments for why the async keyword is a good thing, but none of them actually mentions the real reason why it had to be added to the spec.

The reason is that this was a valid JS pre-ES7

function await(x) {
  return 'awaiting ' + x
}

function foo() {
  return(await(42))
}

According to your logic, would foo() return Promise{42} or "awaiting 42"? (returning a Promise would break backward compatibility)

So the answer is: await is a regular identifier and it's only treated as a keyword inside async functions, so they have to be marked in some way.

Fun fact: the original spec proposed more lightweight function^ foo() {} for async syntax.

1
votes

Because using await inside middleware function means the middleware function can't return a result immediately (it must wait until await is settled) and middleware function callers must wait until the promise (returned from middleware function) is settled.