Embedding Node.js ESM

[segevfiner]

The current embedding API gives you to ability to run a CJS script (That seems to need to bootstrap require by itself), or to get a callback with a runCjs function, but what's the right way to run ESM when embedding? The current sample https://nodejs.org/docs/latest-v20.x/api/embedding.html uses runInThisContext which doesn't support dynamic import by default without vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER which dumps an experimental warning, and vm.SourceTextModule is experimental, which I'm not sure how to correctly enable when embedding, and also seems incomplete such as requiring a link callback without giving you access to the default resolution algorithm, and so on... And the CJS main execution is sync, so if you import, how would you wait for the resulting promise to finish from embedding code?

[DavidPeicho]

I worked around that by doing something like that in the warmup script:

globalThis._loadModuleESM = async function (source, url) {
    const {loadESM} = require('internal/process/esm_loader');

    let result = null;
    return loadESM((loader) => {
        result = loader.eval(source, url).catch((e) => {
            /* Wrap non-native errors to simplify error-detection. */
            if (!(e instanceof Error)) e = new Error(e);
            return e;
        });
        return result;
    }).then(() => result);
};

Then you can retrieve _loadModuleESM to load ESM modules on the C++ side. Don't forget to call this method using MakeCallback, and to spin the event loop.

It's not the best, but at least it works.