RFE: ability to elide the automatic "export default ..." for ES6 module builds

[sgbeal]

When building an ES6 module, Emscripten automatically adds:

export default myModuleInitFunction;

to the resulting JS file. "The problem" with that is that we (the sqlite project) have to, due in part to #18071, overwrite the module init function object with a modified one. We do this via --extern-post-js and then want to do:

export default ourModifiedInitFunction;

but it's illegal to do "export default" twice in the same module.

Our current workaround is this bit of hackery in our makefile after compiling sqlite3.mjs:

	@if [ esm = $(1) ]; then \
		echo "Fragile workaround for an Emscripten annoyance. See emcc.flags.sqlite3.esm."; \
		sed -i -e '0,/^export default/{/^export default/d}' $@ || exit $$?; \
		if ! grep -q '^export default' $@; then \
			echo "Cannot find export default." 1>&2; \
			exit 1; \
		fi; \
	fi

i.e. we delete the first "export default" line, leaving ours intact. That's horribly fragile.

Sidebar: the auto-injected "export default" line does not include a newline after it, which causes --extern-post-js to be appended starting on that same line, which requires a separate kludge on our side to account for.

What would be really useful is a way to tell emcc to not add the "export default" line automatically. When building an .mjs file it automatically does so even if -sEXPORT_ES6=0 is used. An alternative implementation would be to not add that line when -sEXPORT_ES6=0, but whether or not that would have other side effects is a question i cannot answer.

[simonbuehler]

when building opencv.js with "export default --build_flags="-s EXPORT_ES6=1 " also this code gets generated:

var cv = (() => {
  var _scriptDir = import.meta.url;
  
  return (
async function(moduleArg = {}) {

function GROWABLE_HEAP_I8(){if(wasmMemory.buffer!=HEAP8.buffer){updateMemoryViews()}return HEAP8}funct ......

  return moduleArg.ready
}
);
})();
;
export default cv;
  if (typeof Module === 'undefined')
    Module = {};
  return cv(Module);
}));
        

here two problems do exist:

• export default cv; is invalid at this position, should be last line in file ( outer scope)
• Module = {}; should be varModule = {}; so vite does accept the build

[sbc100]

@sgbeal could you say more about why you need to overwrite the module init function object with a modified one?

[sgbeal]

@sgbeal could you say more about why you need to overwrite the module init function object with a modified one?

That's described in #18071.