Ill-formed exports field lacks clear error message when using ESM and self-resolve

[aduh95]

• Version: v14.0.0-pre (master)
• Platform: any
• Subsystem: esm

Let's take this package.json:

{
  "name":"pkgname",
  "exports" : {
    "import": "./file.mjs",
    "require": "./file.cjs",
    "./custom-export" : "./another/file.js"
  }
}

When I try to use a self-resolve import, the error I get doesn't give any clue on why it has failed:

$ echo "import 'pkgname'" > entry.mjs
$ node entry.mjs
(node:95454) ExperimentalWarning: The ESM module loader is experimental.
internal/modules/esm/resolve.js:58
  let url = moduleWrapResolve(specifier, parentURL);
            ^

Error: Cannot find package 'pkgname' imported from .../entry.mjs
    at Loader.defaultResolve [as _resolve] (internal/modules/esm/resolve.js:58:13)
    at Loader.resolve (internal/modules/esm/loader.js:85:40)
    at Loader.getModuleJob (internal/modules/esm/loader.js:188:40)
    at ModuleWrap.<anonymous> (internal/modules/esm/module_job.js:42:40)
    at link (internal/modules/esm/module_job.js:41:36) {
  code: 'ERR_MODULE_NOT_FOUND'
}

When using require, the error is quite clear on what the problem is and what solution to apply:

$ node -e "require('pkgname')"
internal/modules/cjs/loader.js:481
      throw new ERR_INVALID_PACKAGE_CONFIG(basePath, '"exports" cannot ' +
      ^

Error [ERR_INVALID_PACKAGE_CONFIG]: Invalid package config for '...', "exports" cannot contain some keys starting with '.' and some not. The exports object must either be an object of package subpath keys or an object of main entry condition name keys only.
    at isConditionalDotExportSugar (internal/modules/cjs/loader.js:481:13)
    at applyExports (internal/modules/cjs/loader.js:497:7)
    at trySelf (internal/modules/cjs/loader.js:451:23)
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:1013:22)
    at Function.Module._load (internal/modules/cjs/loader.js:913:27)
    at Module.require (internal/modules/cjs/loader.js:1097:19)
    at require (internal/modules/cjs/helpers.js:72:18)
    at [eval]:1:1
    at Script.runInThisContext (vm.js:120:20)
    at Object.runInThisContext (vm.js:311:38) {
  code: 'ERR_INVALID_PACKAGE_CONFIG'
}

Also, I find the documentation not very clear when it comes to package.json formatting; there are some information on the ES Modules page, and a (somewhat outdated) guide on the nodejs.org website, but other than that, I failed to find a clear explanation on how I can make self resolve work (other than the error message).

CC @jkrems

[jkrems]

Thanks for the report! We should definitely aim for parity in these error messages. This sounds like the error message isn't specifically about self resolve but about exports. I assume the same behavior would apply when trying to import the package from the outside..?

[...] but other than that, I failed to find a clear explanation on how I can make self resolve work (other than the error message).

Yeah, unfortunately the docs are still lacking in terms of proper guides. The TL;DR of self resolve is: It should roughly work as if the package was installed as a dependency of itself, as long as the package is using exports. But the only thing we really document right now is how it integrates into the overall resolution algorithm: https://nodejs.org/api/modules.html#modules_all_together

[aduh95]

I assume the same behavior would apply when trying to import the package from the outside..?

No it seems to be related to self-resolve; I moved the test repository in a node_modules folder, and try to to import it, I got a clearer error message:

$ mkdir -p node_modules/pkgname
$ mv package.json node_modules/pkgname
$ node entry.mjs
(node:74691) ExperimentalWarning: The ESM module loader is experimental.
internal/modules/esm/resolve.js:58
  let url = moduleWrapResolve(specifier, parentURL);
            ^

SyntaxError: Cannot resolve package exports in .../node_modules/pkgname/package.json, imported from .../entry.mjs. "exports" cannot contain some keys starting with '.' and some not. The exports object must either be an object of package subpath keys or an object of main entry condition name keys only.
    at Loader.defaultResolve [as _resolve] (internal/modules/esm/resolve.js:58:13)
    at Loader.resolve (internal/modules/esm/loader.js:85:40)
    at Loader.getModuleJob (internal/modules/esm/loader.js:188:40)
    at ModuleWrap.<anonymous> (internal/modules/esm/module_job.js:42:40)
    at link (internal/modules/esm/module_job.js:41:36) {
  code: 'ERR_INVALID_PACKAGE_CONFIG'
}

[aduh95]

Fix landed in 58de9b4.