Top-level await does not work with dynamic await import()

[TooTallNate]

// mod.ts
const sleep = (n: number) => new Promise(r => setTimeout(r, n));

await sleep(100);

export default 1;

// run.ts

// Works
// import mod from './mod.ts';
// console.log(mod);

// Doesn't work:
//
//   error: Uncaught ReferenceError: default is not defined
const mod = await import('./mod.ts');
console.log(mod);

// Also does not work
//
// import('./mod.ts').then(mod => {
//   console.log(mod);
// });

Running:

$ deno run --allow-read run.ts
error: Uncaught ReferenceError: default is not defined
    at Function.keys (<anonymous>)
    at inspectRawObject (rt/02_console.js:675:31)
    at inspectObject (rt/02_console.js:778:14)
    at inspectValue (rt/02_console.js:429:16)
    at inspectArgs (rt/02_console.js:877:16)
    at Object.log (rt/02_console.js:918:9)
    at file:///Users/nrajlich/deno-bug/run.ts:8:9

Seems like the import() function is not properly waiting for the module to become resolved, so there is no default property on the object returned from import().

Initially reported in: vercel-community/deno#40

[lucacasonato]

Hey @MylesBorins - can you take a look at this issue? How are TLA and dynamic imports meant to interact? Should this example work? Thanks :-)

[MylesBorins]

I can confirm that one should be able to await a dynamic import, that was one of the motivating use cases in the original proposal (although not to be abused, please still use static import too!)

I can confirm that the same code, with slight modifications, works in node.js v14.12.0

[kitsonk]

Yeah, I don't think it is TLA. As @TooTallNate mentions, the module isn't ready when the promise is resolved even using the .then(). Clearly we have something messed up in the module instantiation bit with v8, where we are prematurely resolving the promise. I haven't looked in detail but if IIRC, module instantiation is two pass, and I suspect we are resolving the promise for the importing module, and allowing it to be instantiated when only the dependency analysis phase is completed and not waiting for the instantiation.

[nayeemrmn]

Yeah, I don't think it is TLA. As @TooTallNate mentions, the module isn't ready when the promise is resolved even using the .then().

The issue refers to the TLA in the imported module, I'm pretty sure. The dynamic import promise resolves prematurely when the imported module's execution hits the TLA for the timeout. This might be reproducible with static imports...

Also, it seems it's only reproduced by awaiting a new macrotask or more 😕

[kitsonk]

Ah yeah, I see... The real bug is, dynamically importing a module which has TLA fails (which is exactly what the title says 😱 ). So it feels like we are resolving promises out of order then.

[nuxodin]

I think this is the same:
#5215 "nested top level await dynamic imports broken"

[bartlomieju]

Me and @piscisaureus investigated the issue and arrive at the conclusion that deno_core's module loading need a bigger overhaul to fix this problem. I'll be working towards it.

[TooTallNate]

Should this be re-opened considering that #7672 was reverted?