Incorrect lexing of TLA await in try...catch

[kitsonk]

TypeScript Version: 3.8.3

Search Terms: top level await, try catch

Code

try {
  await (async () => {})(); // TS2304 cannot find name 'await'
} catch {
  //
}

Expected behavior:

The file is considered valid.

Actual behavior:

The file has an error at await of "TS2304 cannot find name 'await'".

Playground Link: https://www.typescriptlang.org/play?#code/C4JwngBA3gUAkAQwO4IJbAgCgQZzAOwGMsBKCAXgD5oBfEzEgbhhokIWEIAtp4B6PixhA

[lissein]

Hi,

This seems to be a problem with all the top level statements of the form await (async () => { ... })() and not only in try...catch.

I'm gonna work on it.

[Shayan-To]

Today I was adding some code to a function and needed to make the function async. But I accidentally put the async keyword on the wrong function and my code became something like this:

function f() {
    await(async () => 0)();
}

And typescript gives me this strange error:

a.ts:2:5 - error TS2304: Cannot find name 'await'.

2     await(async () => 0)();
      ~~~~~


Found 1 error.

And I'm like WHAT THE??? WHAAAT?? WHY?? And I search for the error message and I find this issue, and I'm very very confused for a long time, thinking that typescript does not support syntax like await (async () => 0)() and I say to myself very loudly "WHY in the world would such a compiler not support such syntax..."

I now realize that I have put the async keyword on the wrong function, and I've now learned (the strange way,) that you actually CAN define a variable named await, but I think that the error message in such a case as above should not be something like Cannot find name 'await'....

Update: And now, after writing this, I'm looking at this issue again, and I realize that typescript actually does not support such syntax in some cases. Seems like I only found a case that the support is there, but a bad error message made me think it was not. And I'm now thinking WHY would fixing this issue be postponed for such a long time...

[rbuckton]

This was fixed in TypeScript 4.0: https://www.typescriptlang.org/play?target=99&ts=4.0.5#code/C4JwngBA3gUAkAQwO4IJbAgCgQZzAOwGMsBKCAXgD5oBfEzEgbhhokIWEIAtp4B6PixgwApgA8ADgHsQGKDUZA

[rbuckton]

[...] but I think that the error message in such a case as above should not be something like Cannot find name 'await'....

Outside of an async function or the top level of a module, await is considered an identifier, so the error isn't wrong, per se. We can consider introducing a better error message for this specific case, considering you might have meant to declare the function as async. We already do that if your operand is not parenthesized: function f() { await 1; } gives you an error and a quick fix to "Add async modifier to containing function". We just don't currently offer that quick fix when we initially parsed it as a call expression.

[rbuckton]

I created #46986 to track the possibility of adding a distinct error message for an ambiguous await, along with providing the relevant quick fix.