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?
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:
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.async
makes asynchronous code easier to parse for transpilers, browsers, tools, and humans. 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()
returnPromise{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.
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。
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