Emscripten: EXPORT_ES6: Add testing and support for node.js (replace require(), __dirname)

[curiousdannii]

So I'm finally getting around to trying EXPORT_ES6 mode, and it just doesn't work in node (when loaded as an actual ES6 Module), because it still refers to __dirname. I assume other people have only been using it Webpack/rollup/whatever?

It doesn't look like there are any tests for EXPORT_ES6 either.

[kripken]

I think we don't have tests for it because at the time there wasn't a version of node that supported it. If there is now, we should add some.

I think we have 12.10 on CI, and can maybe add another version. Another option is to use v8, which we already install a new version of, if it has ES6 module support.

It would be good if someone could write up a test for this and then we can check the options.

[curiousdannii]

Node 12.10 will definitely be enough.

[sbc100]

Just FYI, it looks like out current version of node that we use in emsdk and in all our testing is 12.18.1.

[jvail]

Could this issue be extended to cover also "require" vs "import"? I wonder if replacing all lines in the output such as

var nodeFS = require('fs');`

with

#if EXPORT_ES6
const { default: nodeFS } = await import('fs');
#else
var nodeFS = require('fs');

would be sufficient.

Unfortunately my insight in the build system is very limited. But if someone could start then I could help at least with testing.

[curiousdannii]

Realised this is going to be difficult to handle for multi-environment builds. ES import statements can't be put in conditionals, and while the import function can, it needs to be handled async.

[RReverser]

ES import statements can't be put in conditionals, and while the import function can, it needs to be handled async.

They don't need to, Emscripten could emit static import declarations instead.

[curiousdannii]

I said it would be difficult for multi-environment builds. A static import declaration for node will fail in the browser.

[RReverser]

Ah, right, I see. In that case, the async import sounds fine. The fact that it's async shouldn't be an issue - Emscripten already has a set of async runtime dependencies and a way to wait on them.

Take a look at usages of addRunDependency from https://github.com/emscripten-core/emscripten/blob/0cc4a667828be0793db01c8c6b85434b6ccb38dc/src/preamble.js#L488.