Allow packages to be marked 'external'

[Rich-Harris]

When using Snowpack to build an app that will run in Node (i.e. snowpack build --ssr), I encounter errors like this:

Error: Package "fs" not found. Have you installed it?

A possible solution would be to add an external option that is an array of package names that Snowpack should ignore. For example:

// snowpack.config.js
module.exports = {
  ...,
  external: [
    // ignore `import fs from 'fs'` etc
    ...require('module').builtinModules,

    // ignore things specified in pkg.dependencies, so that
    // only pkg.devDependencies get handled by Snowpack
    // (this can drastically reduce the size of a deployed
    // app, since you can deploy an already-optimized
    // web_modules and a tiny node_modules, instead of
    // having web_modules duplicated in a huge node_modules)
    ...Object.keys(require('./package.json').dependencies)
  ]
};

Since snowpack build --ssr is generally likely to be used alongside snowpack build, I don't think we can just automatically ignore built-in modules in SSR mode (for example in Svelte apps, you're likely to have modules that are only invoked by the server, but which are nevertheless in a directory that is mounted in both SSR and non-SSR scenarios, so Snowpack would hit the same 'not found' problem when building that mounted directory in non-SSR mode) — it probably needs user control.

[FredKSchott]

This is a great call-out, we actually have config to support this but since an external import would almost always fail in the browser we've never really documented it in Snowpack. You can see the full list of supported installOptions here: https://www.npmjs.com/package/esinstall#api

This should work:

// snowpack.config.js
module.exports = {
  installOptions: {
    externalPackage: [
    // ignore `import fs from 'fs'` etc
    ...require('module').builtinModules,
    // ignore things specified in pkg.dependencies, so that
    // only pkg.devDependencies get handled by Snowpack
    // (this can drastically reduce the size of a deployed
    // app, since you can deploy an already-optimized
    // web_modules and a tiny node_modules, instead of
    // having web_modules duplicated in a huge node_modules)
    ...Object.keys(require('./package.json').dependencies)
    ]
  }
};

[Rich-Harris]

It won't get served to the browser — technically you could import it, but the idea is that these are modules that only do something on the server; we just don't have a good mechanism for excluding them from the directory that is mounted in both contexts. I would expect the untouched import to result in a failed to resolve module specifier 'fs' error if it did somehow end up in the browser.

[FredKSchott]

Okay, I can reproduce in the example kit repo that you shared with offline, but when I add debug logging to Snowpack, it looks like we're sending back "fs" as expected. Are you doing any work on your end that would convert "fs" to an absolute URL/ (turning it into "/_app/routes/fs" in the process?

I can confirm this by adding import "fs" to .svelte/main/App.svelte and then hitting http://localhost:3000/_app/main/App.js?ssr=1 directly and seeing "fs" as-is.

[Rich-Harris]

ohhhh, i wonder if this is caused by our code for loading Snowpacked module graphs in Node: https://github.com/sveltejs/kit/blob/master/packages/kit/src/api/dev/loader.ts#L121-L122

if so I will have egg on my face! checking now...

[Rich-Harris]

looks like this fixed it! sveltejs/kit@3f8cdf0. thank you

[FredKSchott]

great!