JS 异步/等待 - 为什么等待需要异步？

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.

Answer 2

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.

Answer 3

因为在middleware函数中使用await意味着middleware函数不能立即返回结果（它必须等到await被解决）并且middleware函数调用者必须等到承诺（从middleware函数返回）被解决。