loader: implement package maps

Author: arcanis

doc/api/cli.md

@@ -1218,6 +1218,27 @@ added:
 
 Enable experimental support for the network inspection with Chrome DevTools.
 
+### `--experimental-package-map=<path>`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+Enable experimental package map resolution. The `path` argument specifies the
+location of a JSON configuration file that defines package resolution mappings.
+
+```bash
+node --experimental-package-map=./package-map.json app.js
+```
+
+When enabled, bare specifier resolution consults the package map for resolution.
+This allows explicit control over which packages can import which dependencies.
+
+See [Package maps][] for details on the configuration file format and
+resolution algorithm.
+
 ### `--experimental-print-required-tla`
 
 <!-- YAML
@@ -3645,6 +3666,7 @@ one is included in the list below.
 * `--experimental-json-modules`
 * `--experimental-loader`
 * `--experimental-modules`
+* `--experimental-package-map`
 * `--experimental-print-required-tla`
 * `--experimental-quic`
 * `--experimental-require-module`
@@ -4241,6 +4263,7 @@ node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
 [Navigator API]: globals.md#navigator
 [Node.js issue tracker]: https://github.com/nodejs/node/issues
 [OSSL_PROVIDER-legacy]: https://www.openssl.org/docs/man3.0/man7/OSSL_PROVIDER-legacy.html
+[Package maps]: packages.md#package-maps
 [Permission Model]: permissions.md#permission-model
 [REPL]: repl.md
 [ScriptCoverage]: https://chromedevtools.github.io/devtools-protocol/tot/Profiler#type-ScriptCoverage

doc/api/errors.md

@@ -2496,6 +2496,77 @@ A given value is out of the accepted range.
 The `package.json` [`"imports"`][] field does not define the given internal
 package specifier mapping.
 
+<a id="ERR_PACKAGE_MAP_EXTERNAL_FILE"></a>
+
+### `ERR_PACKAGE_MAP_EXTERNAL_FILE`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+A module attempted to resolve a bare specifier using the [package map][], but
+the importing file is not located within any package defined in the map.
+
+```console
+$ node --experimental-package-map=./package-map.json /tmp/script.js
+Error [ERR_PACKAGE_MAP_EXTERNAL_FILE]: Cannot resolve "dep-a" from "/tmp/script.js": file is not within any package defined in /path/to/package-map.json
+```
+
+To fix this error, ensure the importing file is inside one of the package
+directories listed in the package map, or add a new package entry whose `path`
+covers the importing file.
+
+<a id="ERR_PACKAGE_MAP_INVALID"></a>
+
+### `ERR_PACKAGE_MAP_INVALID`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+The [package map][] configuration file is invalid. This can occur when:
+
+* The file does not exist at the specified path.
+* The file contains invalid JSON.
+* The file is missing the required `packages` object.
+* A package entry is missing the required `path` field.
+* Two package entries have the same `path` value.
+
+```console
+$ node --experimental-package-map=./missing.json app.js
+Error [ERR_PACKAGE_MAP_INVALID]: Invalid package map at "./missing.json": file not found
+```
+
+<a id="ERR_PACKAGE_MAP_KEY_NOT_FOUND"></a>
+
+### `ERR_PACKAGE_MAP_KEY_NOT_FOUND`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+A package's `dependencies` object in the [package map][] references a package
+key that is not defined in the `packages` object.
+
+```json
+{
+  "packages": {
+    "app": {
+      "path": "./app",
+      "dependencies": {
+        "foo": "nonexistent"
+      }
+    }
+  }
+}
+```
+
+In this example, `"nonexistent"` is referenced as a dependency target but not
+defined in `packages`, which will throw this error.
+
+To fix this error, ensure all package keys referenced in `dependencies` values
+are defined in the `packages` object.
+
 <a id="ERR_PACKAGE_PATH_NOT_EXPORTED"></a>
 
 ### `ERR_PACKAGE_PATH_NOT_EXPORTED`
@@ -4470,6 +4541,7 @@ An error occurred trying to allocate memory. This should never happen.
 [domains]: domain.md
 [event emitter-based]: events.md#class-eventemitter
 [file descriptors]: https://en.wikipedia.org/wiki/File_descriptor
+[package map]: packages.md#package-maps
 [relative URL]: https://url.spec.whatwg.org/#relative-url-string
 [self-reference a package using its name]: packages.md#self-referencing-a-package-using-its-name
 [special scheme]: https://url.spec.whatwg.org/#special-scheme

doc/api/esm.md

@@ -943,6 +943,12 @@ The default loader has the following properties
 * Fails on unknown extensions for `file:` loading
   (supports only `.cjs`, `.js`, and `.mjs`)
 
+When the [`--experimental-package-map`][] flag is enabled, bare specifier
+resolution first consults the package map configuration. If the importing
+module is within a mapped package and the specifier matches a declared
+dependency, the package map resolution takes precedence. See [Package maps][]
+for details.
+
 ### Resolution algorithm
 
 The algorithm to load an ES module specifier is given through the
@@ -1306,12 +1312,14 @@ resolution for ESM specifiers is [commonjs-extension-resolution-loader][].
 [Loading ECMAScript modules using `require()`]: modules.md#loading-ecmascript-modules-using-require
 [Module customization hooks]: module.md#customization-hooks
 [Node.js Module Resolution And Loading Algorithm]: #resolution-algorithm-specification
+[Package maps]: packages.md#package-maps
 [Source Phase Imports]: https://github.com/tc39/proposal-source-phase-imports
 [Terminology]: #terminology
 [URL]: https://url.spec.whatwg.org/
 [WebAssembly JS String Builtins Proposal]: https://github.com/WebAssembly/js-string-builtins
 [`"exports"`]: packages.md#exports
 [`"type"`]: packages.md#type
+[`--experimental-package-map`]: cli.md#--experimental-package-mappath
 [`--input-type`]: cli.md#--input-typetype
 [`data:` URLs]: https://developer.mozilla.org/en-US/docs/Web/URI/Reference/Schemes/data
 [`export`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export

doc/api/modules.md

@@ -356,8 +356,12 @@ require(X) from module at path Y
 4. If X begins with '#'
    a. LOAD_PACKAGE_IMPORTS(X, dirname(Y))
 5. LOAD_PACKAGE_SELF(X, dirname(Y))
-6. LOAD_NODE_MODULES(X, dirname(Y))
-7. THROW "not found"
+6. If a package map PACKAGE_MAP exists,
+   a. Find the package ID for the package owning Y
+        1. Let PARENT_PACKAGE_ID be FIND_PACKAGE_ID(dirname(Y), PACKAGE_MAP)
+   b. LOAD_PACKAGE_MAP(X, PARENT_PACKAGE_ID, PACKAGE_MAP)
+7. LOAD_NODE_MODULES(X, dirname(Y))
+8. THROW "not found"
 
 MAYBE_DETECT_AND_LOAD(X)
 1. If X parses as a CommonJS module, load X as a CommonJS module. STOP.
@@ -402,9 +406,11 @@ LOAD_AS_DIRECTORY(X)
 2. LOAD_INDEX(X)
 
 LOAD_NODE_MODULES(X, START)
-1. let DIRS = NODE_MODULES_PATHS(START)
-2. for each DIR in DIRS:
-   a. LOAD_PACKAGE_EXPORTS(X, DIR)
+1. Try to interpret X as a combination of NAME and SUBPATH where the name
+   may have a @scope/ prefix and the subpath begins with a slash (`/`).
+2. let DIRS = NODE_MODULES_PATHS(START)
+3. for each DIR in DIRS:
+   a. LOAD_PACKAGE_EXPORTS(SUBPATH, DIR/NAME)
    b. LOAD_AS_FILE(DIR/X)
    c. LOAD_AS_DIRECTORY(DIR/X)
 
@@ -419,6 +425,25 @@ NODE_MODULES_PATHS(START)
    d. let I = I - 1
 5. return DIRS + GLOBAL_FOLDERS
 
+FIND_PACKAGE_ID(PATH, PACKAGE_MAP)
+1. Find the PACKAGE_ID for the entry whose "path" is a parent directory of PATH
+2. If multiple entries are found, THROW "ambiguous resolution"
+3. If no entry was found, THROW "external file".
+4. return PACKAGE_ID
+
+LOAD_PACKAGE_MAP(X, PARENT_PACKAGE_ID, PACKAGE_MAP)
+1. Try to interpret X as a combination of NAME and SUBPATH where the name
+   may have a @scope/ prefix and the subpath begins with a slash (`/`).
+2. Find the package map entry for key PARENT_PACKAGE_ID
+3. Look up NAME in the entry's "dependencies" map.
+4. If NAME is not found, THROW "not found".
+5. Let TARGET be PACKAGE_MAP.packages[dependencies[name]]
+6. Let PACKAGE_PATH be the resolved path of TARGET.
+7. LOAD_PACKAGE_EXPORTS(SUBPATH, PACKAGE_PATH)
+8. LOAD_AS_FILE(PACKAGE_PATH/SUBPATH)
+9. LOAD_AS_DIRECTORY(PACKAGE_PATH/SUBPATH)
+10. THROW "not found"
+
 LOAD_PACKAGE_IMPORTS(X, DIR)
 1. Find the closest package scope SCOPE to DIR.
 2. If no scope was found, return.
@@ -430,19 +455,15 @@ LOAD_PACKAGE_IMPORTS(X, DIR)
   CONDITIONS) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
 6. RESOLVE_ESM_MATCH(MATCH).
 
-LOAD_PACKAGE_EXPORTS(X, DIR)
-1. Try to interpret X as a combination of NAME and SUBPATH where the name
-   may have a @scope/ prefix and the subpath begins with a slash (`/`).
-2. If X does not match this pattern or DIR/NAME/package.json is not a file,
-   return.
-3. Parse DIR/NAME/package.json, and look for "exports" field.
-4. If "exports" is null or undefined, return.
-5. If `--no-require-module` is not enabled
+LOAD_PACKAGE_EXPORTS(SUBPATH, PACKAGE_DIR)
+1. Parse PACKAGE_DIR/package.json, and look for "exports" field.
+2. If "exports" is null or undefined, return.
+3. If `--no-require-module` is not enabled
   a. let CONDITIONS = ["node", "require", "module-sync"]
   b. Else, let CONDITIONS = ["node", "require"]
-6. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(DIR/NAME), "." + SUBPATH,
+4. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(PACKAGE_DIR), "." + SUBPATH,
    `package.json` "exports", CONDITIONS) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
-7. RESOLVE_ESM_MATCH(MATCH)
+5. RESOLVE_ESM_MATCH(MATCH)
 
 LOAD_PACKAGE_SELF(X, DIR)
 1. Find the closest package scope SCOPE to DIR.