fix: allow resolving package.json within packages

[paralin]

Enables the following type of import:

const pkg = require('@mantine/core/package.json');

This is used to access the contents of package.json which is usually not exported by any of the exports within the package.json.

Reproduction of the issue:

package main

import (
	"errors"

	esbuild_api "github.com/evanw/esbuild/pkg/api"
)

func main() {
	res := esbuild_api.Build(esbuild_api.BuildOptions{
		EntryPoints: []string{"entry.js"},

		LogLevel: esbuild_api.LogLevelVerbose,
		Platform: esbuild_api.PlatformBrowser,
		Format:   esbuild_api.FormatESModule,

		Bundle:  true,
		Write:   true,
		Outfile: "out.js",

		Plugins: []esbuild_api.Plugin{buildPlugin()},
		Loader: map[string]esbuild_api.Loader{
			".json": esbuild_api.LoaderFile,
		},
	})
	if len(res.Errors) != 0 {
		panic(res.Errors[0].Text)
	}
}

func buildPlugin() esbuild_api.Plugin {
	return esbuild_api.Plugin{
		Name: "logger",
		Setup: func(pb esbuild_api.PluginBuild) {
			pb.OnResolve(esbuild_api.OnResolveOptions{
				Filter:    ".",
				Namespace: "file",
			}, func(ora esbuild_api.OnResolveArgs) (esbuild_api.OnResolveResult, error) {
				var result esbuild_api.OnResolveResult
				if ora.Importer == "logger" {
					return result, nil
				}

				resResult := pb.Resolve("@mantine/core/package.json", esbuild_api.ResolveOptions{
					// Importer: ora.Importer,
					// Namespace:  ora.Namespace,
					// Namespace:  "logger",
					Namespace:  "file",
					Importer:   "logger",
					ResolveDir: ora.ResolveDir,
					Kind:       esbuild_api.ResolveJSImportStatement,
				})
				if len(resResult.Errors) != 0 {
					return result, errors.New(resResult.Errors[0].Text)
				}

				return result, nil
			})
		},
	}
}

Importing package.json currently works with some packages (like react) but not others (like @mantine/core).

[evanw]

The design of the exports feature intentionally makes it the package author’s decision which parts of their package are exported. Package consumers aren’t supposed to be able to override the package author’s decisions if they don’t like them. Sorry but that’s not how the exports feature is supposed to work. You can read the specification for yourself here. I’m closing this issue because it goes against the specification.

[paralin]

@evanw So it's impossible to import package.json? Why? It's in node_modules and always present. Regardless of what the developer exported.

If this isn't possible, is there any way at all to determine the root path (path to package.json parent dir) for an import? Because that seems impossible with esbuild and the above reasoning you have given seems arbitrary and unnecessarily restrictive.

Who does esbuild serve? The developer, or the person who published the package?...

[evanw]

The reason esbuild does this is because it tries to follow already-established specifications and community conventions to have better interoperability with other tools. In this case the node team is the one who created the exports field in package.json so they get to say how it works (which they have said very precisely in a detailed specification). Other tools that interpret exports also all have the same behavior as esbuild here:

• esbuild

  ✘ [ERROR] Could not resolve "@mantine/core/package.json"
  
      index.js:1:20:
        1 │ const pkg = require('@mantine/core/package.json');
          ╵                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
    The path "./package.json" is not exported by package "@mantine/core":
  
      node_modules/@mantine/core/package.json:8:13:
        8 │   "exports": {
          ╵              ^
  

• node

  node:internal/modules/cjs/loader:553
        throw e;
        ^
  
  Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './package.json' is not defined by "exports" in ./node_modules/@mantine/core/package.json
      at new NodeError (node:internal/errors:405:5)
      at exportsNotFound (node:internal/modules/esm/resolve:260:10)
      at packageExportsResolve (node:internal/modules/esm/resolve:590:9)
      at resolveExports (node:internal/modules/cjs/loader:547:36)
      at Module._findPath (node:internal/modules/cjs/loader:621:31)
      at Module._resolveFilename (node:internal/modules/cjs/loader:1034:27)
      at Module._load (node:internal/modules/cjs/loader:901:27)
      at Module.require (node:internal/modules/cjs/loader:1115:19)
      at require (node:internal/modules/helpers:119:18)
      at Object.<anonymous> (./index.js:1:13) {
    code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
  }
  

• webpack

  assets by status 267 bytes [cached] 1 asset
  ./index.js 50 bytes [built] [code generated]
  
  ERROR in ./index.js 1:12-49
  Module not found: Error: Package path ./package.json is not exported from package ./node_modules/@mantine/core (see exports field in ./node_modules/@mantine/core/package.json)
  

• rollup

  index.js → stdout...
  import '@mantine/core/package.json';
  
  var temp = {};
  
  export { temp as default };
  (!) Plugin node-resolve: Could not resolve import "@mantine/core/package.json" in ./index.js using exports defined in ./node_modules/@mantine/core/package.json.
  (!) Plugin node-resolve: Could not resolve import "@mantine/core/package.json" in undefined using exports defined in ./node_modules/@mantine/core/package.json.
  (!) Unresolved dependencies
  

Who does esbuild serve? The developer, or the person who published the package?...

According to the specification for exports, in this situation esbuild is supposed to serve the person who published the package instead of the person using the package. Here you can see where the node developers have considered and rejected the same exception to the specification that you are proposing: nodejs/node#33460. So you are unlikely to be able to convince node to change the specification. Instead, I recommend reaching out to the author of that library to get them to allow access to package.json if they are open to it.

[paralin]

@evanw I understand the reasoning and the link you provided helps to clarify the situation quite a bit, thanks.