docs for --pre-js

[Brian Craft]

I haven't been able to find docs or examples for --pre-js. Can anyone point in the right direction?

I worked out that it's "--pre-js=foo.js", not "--pre-js foo.js". I don't understand the module system, though, and can't access the functions defined in the file.

I was expecting that I would add the functions to EXTRA_EXPORTED_RUNTIME_METHODS, and they would appear on Module. This seems to sort of half work: if I add a function to EXTRA_EXPORTED_RUNTIME_METHODS but fail to set --pre-js, the build will cough up an error, like

shared:ERROR: undefined exported function: "fradixSortL16_64"

After adding --pre-js, this build error disappears, so it seems to be finding the function during build, but the function is not available at run time on Module.

[Brian Craft]

On review, this isn't quite correct. It wasn't --pre-js that fixed the build error, it was moving the export from EXPORTED_FUNCTIONS to EXTRA_EXPORTED_RUNTIME_METHODS. So, I don't have any evidence that it's finding the methods in the --pre-js file. Adding random names to the runtime method list doesn't generate any build-time error.

[Alon Zakai]

The docs are shown with `emcc --help`, also available here

https://emscripten.org/docs/tools_reference/emcc.html

The syntax is a space, so --pre-js FILE.js

See the test suite for working examples of all this stuff, can search for `--pre-js` in tests/*.py.

If you want the function to be exportable on Module, then it's better to use `--js-library`, btw. Such functions are just normal JS system libraries, exportable in the usual way.

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