javascript - JS async/await - why does await need async?

Question

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?

Answer 1

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.