Support passthrough dynamic import, or basic code doesn't work

[AshleyScirra]

Please at least support a basic passthrough for dynamic import for Closure Compiler. In the current version, any presence of dynamic import at all causes a compilation failure. This causes even basic use cases of dynamic import to cause the entire compilation to fail.

Example in.js:

(async () =>
{
	await import("https://cdn.example.com/library.js");
	externalLibrary.someFunc();
})();

Command line:

java -jar ./closure-compiler.jar --js in.js --js_output_file out.js --compilation_level SIMPLE --formatting PRETTY_PRINT --language_in ECMASCRIPT_2020 --language_out ECMASCRIPT_2020

Observed result:

in.js:4:7: WARNING - [JSC_PARSE_ERROR] Parse error. This language feature is not currently supported by the compiler: Dynamic module import
  4|    await import("https://cdn.example.com/library.js");
              ^

ERROR - [JSC_FEATURES_NOT_SUPPORTED_BY_PASS] Attempted to run pass "markUntranspilableFeaturesAsRemoved" on input with features it does not support. Running pass anyway.
Unsupported features: [Dynamic module import]

1 error(s), 1 warning(s)

No output file is generated.

Expected result:

Just output the file with all transforms, but passthrough import(XYZ) -> import(XYZ)

This is blocking us using JavaScript modules at all in our code. We want to insert a dynamic import, but then Closure Compiler will refuse to process any of our code at all.

[AshleyScirra]

The same goes for import.meta too - we also need to use import.meta.url, but the fact all compilation fails if it's present anywhere is a major limitation. That should be able to passthrough too. We know the url or other metadata might change after compilation; that's fine, it either changes nothing in our case or is something we deal with.

[concavelenz]

We need some way to distinguish between imports that should be rewritten and those that folks want to preserve.

[AshleyScirra]

Just want to link to this comment here as well - for the "just minify" case I think it would be fine to have a flag that just says "pass through all imports/modules features without changing them".

[ctjlewis]

Yeah, it would be great if I could tell the compiler not to worry about import.meta. @supress or @external import.meta would be good approaches - tried both, compiler won't take the source if it contains an import.meta declaration.

[brad4d]

Note that there is already an open issue for dynamic import support

#2770

I guess this one is asking for something a bit more targeted, but I'm still a bit inclined to close this as a duplicate so all the discussion will be in one place.

@AshleyScirra wdyt?

[AshleyScirra]

I think #2770 is asking for full support. I'm only asking here for Closure Compiler not to fail compilation due to the presence of dynamic import, i.e. the minimum possible support. I think there's basically three levels of support:

1. Fail compilation if feature exists (current state)
2. Ignore feature/pass through compilation without alteration (what I'm asking for here)
3. Full support (e.g. potentially taking in to account dead code elimination, bundling, transpilation, tree shaking etc) - as apparently described in issue [FEATURE] Support dynamic import #2770

I'd have thought passthrough is more or less trivial, whereas full support would involve more significant work.

[brad4d]

@AshleyScirra do you feel this issue would be completely closed by the following behavior?

If the compiler's output is set to at least ES_2020, then:

1. import(anything) will be left in the source code entirely unmodified.
2. Specifically, the compiler will not pay any attention at all to the value passed to import(), so it won't notice if it looks like the path to a file that is included in the compilation.
3. The compiler will infer the type of all such expressions to be Promise<*>. (You must cast the result of the promise to some type the compiler knows about in order to do anything with it.)

[AshleyScirra]

Yes, that sounds like it would do it.

[GreenFootballs]

Has this issue been addressed yet? I've had to revert to an earlier version to compile a project I'm working on, because dynamic import causes the compiler to exit with an error.

[ChadKillingsworth]

There is a PR for basic pass through support under review: #3706.

[GreenFootballs]

Great, thanks. I'll watch for it.

[RReverser]

Looks like PR above addressed dynamic import, but import.meta still fails.

[ChadKillingsworth]

dynamic import and import.meta are now supported

[ChadKillingsworth]

There's a couple of more places that need to support import.meta.