[javascript] Uncaught TypeError: (intermediate value)(...) is not a function

For me it was much more simple but it took me a while to figure it out. We basically had in our .jslib

some_array.forEach(item => {
    do_stuff(item);
});

Turns out Unity (emscripten?) just doesn't like that syntax. We replaced it with a good old for-loop and it stoped complaining right away. I really hate it that it doesn't show the line it is complaining about, but anyway, fool me twice shame on me.