main.c

extern void some_external_JS(int);

int main() {
  some_external_JS(42);
}

main.js

global.Module = {};
global.print = console.log;

require('./a.asm.js'); // evals the script

var asm = global.Module['asm'](global, {
  _some_external_JS: function(x) {
    print('hello, some_external_JS was called with ' + x);
  },
}, new ArrayBuffer(1024*1024));

asm._main();

overview.txt

emcc main.c -o a.html --separate-asm -s NO_FILESYSTEM=1 -Oz -profiling
# ignore html and js, look at just the a.asm.js
# run main.js
# remove -profiling to reduce code size, but asm is harder to read