WIP: import map loader

doc/api/errors.md

@@ -1947,6 +1947,13 @@ for more information.
 
 An invalid HTTP token was supplied.
 
+<a id="ERR_INVALID_IMPORT_MAP"></a>
+
+### `ERR_INVALID_IMPORT_MAP`
+
+An invalid import map file was supplied. This error can throw for a variety
+of conditions which will change the error message for added context.
+
 <a id="ERR_INVALID_IP_ADDRESS"></a>
 
 ### `ERR_INVALID_IP_ADDRESS`

lib/internal/errors.js

@@ -1415,6 +1415,7 @@ E('ERR_INVALID_FILE_URL_HOST',
 E('ERR_INVALID_FILE_URL_PATH', 'File URL path %s', TypeError);
 E('ERR_INVALID_HANDLE_TYPE', 'This handle type cannot be sent', TypeError);
 E('ERR_INVALID_HTTP_TOKEN', '%s must be a valid HTTP token ["%s"]', TypeError);
+E('ERR_INVALID_IMPORT_MAP', 'Invalid import map: %s', Error);
 E('ERR_INVALID_IP_ADDRESS', 'Invalid IP address: %s', TypeError);
 E('ERR_INVALID_MIME_SYNTAX', (production, str, invalidIndex) => {
   const msg = invalidIndex !== -1 ? ` at ${invalidIndex}` : '';

lib/internal/modules/esm/import_map.js

@@ -0,0 +1,107 @@
+'use strict';
+const { isURL, URL } = require('internal/url');
+const { ObjectEntries, ObjectKeys, SafeMap, ArrayIsArray } = primordials;
+const { codes: { ERR_INVALID_IMPORT_MAP } } = require('internal/errors');
+
+class ImportMap {
+  #baseURL;
+  imports = new SafeMap();
+  scopes = new SafeMap();
+
+  constructor(raw, baseURL) {
+    this.#baseURL = baseURL;
+    processImportMap(this, this.#baseURL, raw);
+  }
+
+  get baseURL() {
+    return this.#baseURL;
+  }
+
+  resolve(specifier, parentURL = this.baseURL) {
+    // Process scopes
+    for (const { 0: prefix, 1: mapping } of this.scopes) {
+      let mappedSpecifier = mapping.get(specifier);
+      if (parentURL.pathname.startsWith(prefix.pathname) && mappedSpecifier) {
+        if (!isURL(mappedSpecifier)) {
+          mappedSpecifier = new URL(mappedSpecifier, this.baseURL);
+          mapping.set(specifier, mappedSpecifier);
+        }
+        specifier = mappedSpecifier;
+        break;
+      }
+    }
+
+    let spec = specifier;
+    if (isURL(specifier)) {
+      spec = specifier.pathname;
+    }
+    let importMapping = this.imports.get(spec);
+    if (importMapping) {
+      if (!isURL(importMapping)) {
+        importMapping = new URL(importMapping, this.baseURL);
+        this.imports.set(spec, importMapping);
+      }
+      return importMapping;
+    }
+
+    return specifier;
+  }
+}
+
+function processImportMap(importMap, baseURL, raw) {
+  // Validation and normalization
+  if (typeof raw.imports !== 'object' || ArrayIsArray(raw.imports)) {
+    throw new ERR_INVALID_IMPORT_MAP('top level key "imports" is required and must be a plain object');
+  }
+  if (typeof raw.scopes !== 'object' || ArrayIsArray(raw.scopes)) {
+    throw new ERR_INVALID_IMPORT_MAP('top level key "scopes" is required and must be a plain object');
+  }
+
+  // Normalize imports
+  for (const { 0: specifier, 1: mapping } of ObjectEntries(raw.imports)) {
+    if (!specifier || typeof specifier !== 'string') {
+      throw new ERR_INVALID_IMPORT_MAP('module specifier keys must be non-empty strings');
+    }
+    if (!mapping || typeof mapping !== 'string') {
+      throw new ERR_INVALID_IMPORT_MAP('module specifier values must be non-empty strings');
+    }
+    if (specifier.endsWith('/') && !mapping.endsWith('/')) {
+      throw new ERR_INVALID_IMPORT_MAP('module specifier values for keys ending with / must also end with /');
+    }
+
+    importMap.imports.set(specifier, mapping);
+  }
+
+  // Normalize scopes
+  // Sort the keys according to spec and add to the map in order
+  // which preserves the sorted map requirement
+  const sortedScopes = ObjectKeys(raw.scopes).sort().reverse();
+  for (let scope of sortedScopes) {
+    const _scopeMap = raw.scopes[scope];
+    if (!scope || typeof scope !== 'string') {
+      throw new ERR_INVALID_IMPORT_MAP('import map scopes keys must be non-empty strings');
+    }
+    if (!_scopeMap || typeof _scopeMap !== 'object') {
+      throw new ERR_INVALID_IMPORT_MAP(`scope values must be plain objects (${scope} is ${typeof _scopeMap})`);
+    }
+
+    // Normalize scope
+    scope = new URL(scope, baseURL);
+
+    const scopeMap = new SafeMap();
+    for (const { 0: specifier, 1: mapping } of ObjectEntries(_scopeMap)) {
+      if (specifier.endsWith('/') && !mapping.endsWith('/')) {
+        throw new ERR_INVALID_IMPORT_MAP('module specifier values for keys ending with / must also end with /');
+      }
+      scopeMap.set(specifier, mapping);
+    }
+
+    importMap.scopes.set(scope, scopeMap);
+  }
+
+  return importMap;
+}
+
+module.exports = {
+  ImportMap,
+};

lib/internal/modules/esm/loader.js

@@ -129,6 +129,11 @@ class ModuleLoader {
    */
   #customizations;
 
+  /**
+   * The loaders importMap instance
+   */
+  importMap;
+
   constructor(customizations) {
     if (getOptionValue('--experimental-network-imports')) {
       emitExperimentalWarning('Network Imports');
@@ -391,6 +396,7 @@ class ModuleLoader {
       conditions: this.#defaultConditions,
       importAttributes,
       parentURL,
+      importMap: this.importMap,
     };
 
     return defaultResolve(originalSpecifier, context);

lib/internal/modules/esm/resolve.js

@@ -1026,6 +1026,35 @@ function throwIfInvalidParentURL(parentURL) {
   }
 }
 
+/**
+ * Process policy
+ */
+function processPolicy(specifier, context) {
+  const { parentURL, conditions } = context;
+  const redirects = policy.manifest.getDependencyMapper(parentURL);
+  if (redirects) {
+    const { resolve, reaction } = redirects;
+    const destination = resolve(specifier, new SafeSet(conditions));
+    let missing = true;
+    if (destination === true) {
+      missing = false;
+    } else if (destination) {
+      const href = destination.href;
+      return { __proto__: null, url: href };
+    }
+    if (missing) {
+      // Prevent network requests from firing if resolution would be banned.
+      // Network requests can extract data by doing things like putting
+      // secrets in query params
+      reaction(new ERR_MANIFEST_DEPENDENCY_MISSING(
+        parentURL,
+        specifier,
+        ArrayPrototypeJoin([...conditions], ', ')),
+      );
+    }
+  }
+}
+
 /**
  * Resolves the given specifier using the provided context, which includes the parent URL and conditions.
  * Throws an error if the parent URL is invalid or if the resolution is disallowed by the policy manifest.
@@ -1037,31 +1066,8 @@ function throwIfInvalidParentURL(parentURL) {
  */
 function defaultResolve(specifier, context = {}) {
   let { parentURL, conditions } = context;
+  const { importMap } = context;
   throwIfInvalidParentURL(parentURL);
-  if (parentURL && policy?.manifest) {
-    const redirects = policy.manifest.getDependencyMapper(parentURL);
-    if (redirects) {
-      const { resolve, reaction } = redirects;
-      const destination = resolve(specifier, new SafeSet(conditions));
-      let missing = true;
-      if (destination === true) {
-        missing = false;
-      } else if (destination) {
-        const href = destination.href;
-        return { __proto__: null, url: href };
-      }
-      if (missing) {
-        // Prevent network requests from firing if resolution would be banned.
-        // Network requests can extract data by doing things like putting
-        // secrets in query params
-        reaction(new ERR_MANIFEST_DEPENDENCY_MISSING(
-          parentURL,
-          specifier,
-          ArrayPrototypeJoin([...conditions], ', ')),
-        );
-      }
-    }
-  }
 
   let parsedParentURL;
   if (parentURL) {
@@ -1079,8 +1085,19 @@ function defaultResolve(specifier, context = {}) {
     } else {
       parsed = new URL(specifier);
     }
+  } catch {
+    // Ignore exception
+  }
 
-    // Avoid accessing the `protocol` property due to the lazy getters.
+  // Import maps are processed before policies and data/http handling
+  // so policies apply to the result of any mapping
+  if (importMap) {
+    // Intentionally mutating here as we don't think it is a problem
+    parsed = specifier = importMap.resolve(parsed || specifier, parsedParentURL);
+  }
+
+  // Avoid accessing the `protocol` property due to the lazy getters.
+  if (parsed) {
     const protocol = parsed.protocol;
     if (protocol === 'data:' ||
       (experimentalNetworkImports &&
@@ -1092,8 +1109,13 @@ function defaultResolve(specifier, context = {}) {
     ) {
       return { __proto__: null, url: parsed.href };
     }
-  } catch {
-    // Ignore exception
+  }
+
+  if (parentURL && policy?.manifest) {
+    const policyResolution = processPolicy(specifier, context);
+    if (policyResolution) {
+      return policyResolution;
+    }
   }
 
   // There are multiple deep branches that can either throw or return; instead

lib/internal/modules/run_main.js

@@ -51,6 +51,7 @@ function resolveMainPath(main) {
  */
 function shouldUseESMLoader(mainPath) {
   if (getOptionValue('--experimental-default-type') === 'module') { return true; }
+  if (getOptionValue('--experimental-import-map')) { return true; }
 
   /**
    * @type {string[]} userLoaders A list of custom loaders registered by the user
@@ -92,10 +93,24 @@ function shouldUseESMLoader(mainPath) {
  */
 function runMainESM(mainPath) {
   const { loadESM } = require('internal/process/esm_loader');
-  const { pathToFileURL } = require('internal/url');
+  const { pathToFileURL, URL } = require('internal/url');
+  const _importMapPath = getOptionValue('--experimental-import-map');
   const main = pathToFileURL(mainPath).href;
 
   handleMainPromise(loadESM((esmLoader) => {
+    // Load import map and throw validation errors
+    if (_importMapPath) {
+      const { ImportMap } = require('internal/modules/esm/import_map');
+      const { getCWDURL } = require('internal/util');
+
+      const importMapPath = esmLoader.resolve(_importMapPath, getCWDURL(), { __proto__: null, type: 'json' });
+      return esmLoader.import(importMapPath.url, getCWDURL(), { __proto__: null, type: 'json' })
+        .then((importedMapFile) => {
+          esmLoader.importMap = new ImportMap(importedMapFile.default, new URL(importMapPath.url));
+          return esmLoader.import(main, undefined, { __proto__: null });
+        });
+    }
+
     return esmLoader.import(main, undefined, { __proto__: null });
   }));
 }

src/node_options.cc

@@ -546,6 +546,10 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
             &EnvironmentOptions::prof_process);
   // Options after --prof-process are passed through to the prof processor.
   AddAlias("--prof-process", { "--prof-process", "--" });
+  AddOption("--experimental-import-map",
+            "set the path to an import map.json",
+            &EnvironmentOptions::import_map_path,
+            kAllowedInEnvvar);
 #if HAVE_INSPECTOR
   AddOption("--cpu-prof",
             "Start the V8 CPU profiler on start up, and write the CPU profile "

src/node_options.h

@@ -179,6 +179,7 @@ class EnvironmentOptions : public Options {
   bool extra_info_on_fatal_exception = true;
   std::string unhandled_rejections;
   std::vector<std::string> userland_loaders;
+  std::string import_map_path;
   bool verify_base_objects =
 #ifdef DEBUG
       true;