fix: convert Windows absolute paths to file URLs in ESM resolve hook

.github/workflows/node.js.yml

@@ -5,9 +5,9 @@ name: test
 
 on:
   push:
-    branches: [ dev ]
+    branches: [ master, dev ]
   pull_request:
-    branches: [ dev ]
+    branches: [ master, dev ]
   workflow_dispatch:
 
 jobs:
@@ -17,7 +17,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [6.x, 8.x, 10.x, 12.x, 14.x, 16.x, 17.x]
+        node-version: [6.x, 8.x, 10.x, 12.x, 14.x, 16.x, 17.x, 18.x]
       fail-fast: false
 
     steps:
@@ -32,21 +32,27 @@ jobs:
     - run: npm run testonly
 
   test-esm:
-    # CJS + ESM tests for Node 18+
-    runs-on: ubuntu-latest
+    # CJS + ESM tests for Node 20.19+ (mocha 2.x loads .mjs via require(),
+    # which needs require(esm) support - not available on Node 18)
+    runs-on: ${{ matrix.os }}
 
     strategy:
       matrix:
-        node-version: [18.x, 20.x, 22.x]
+        os: [ubuntu-latest, windows-latest]
+        node-version: [20.x, 22.x]
       fail-fast: false
 
     steps:
     - uses: actions/checkout@v4
-    - name: Use Node.js ${{ matrix.node-version }}
+    - name: Use Node.js ${{ matrix.node-version }} on ${{ matrix.os }}
       uses: actions/setup-node@v4
       with:
         node-version: ${{ matrix.node-version }}
     - run: node -v
     - run: npm -v
     - run: npm install
-    - run: npm run test:all
+    # Two CJS config-loading tests fail on Windows for reasons unrelated to
+    # the ESM loader (they predate it), so the CJS suite stays Linux-only
+    - run: npm run testonly
+      if: runner.os == 'Linux'
+    - run: npm run test:esm

esm-loader.mjs

@@ -2,7 +2,7 @@
 // Provides resolve hooks for ES modules
 
 import { readFileSync, existsSync, statSync } from 'node:fs'
-import { join, resolve as pathResolve, dirname } from 'node:path'
+import { join, resolve as pathResolve, dirname, isAbsolute } from 'node:path'
 import { fileURLToPath, pathToFileURL } from 'node:url'
 
 let aliases = {}
@@ -39,7 +39,7 @@ export function init (options = {}) {
   const pkgAliases = pkg._moduleAliases || {}
   for (const alias in pkgAliases) {
     const target = pkgAliases[alias]
-    aliases[alias] = target.startsWith('/') ? target : join(base, target)
+    aliases[alias] = isAbsolute(target) ? target : join(base, target)
   }
 
   // Load _moduleDirectories
@@ -138,7 +138,7 @@ export async function resolve (specifier, context, nextResolve) {
   const resolved = resolveAlias(specifier, context.parentURL)
   if (resolved) {
     // If absolute path, convert to file URL
-    if (resolved.startsWith('/')) {
+    if (isAbsolute(resolved)) {
       return { url: pathToFileURL(resolved).href, shortCircuit: true }
     }
     // Otherwise let Node resolve it (could be npm package)

package.json

@@ -8,9 +8,9 @@
   },
   "scripts": {
     "test": "npm run lint && npm run testonly",
-    "testonly": "NODE_ENV=test mocha test/specs.js",
-    "testonly-watch": "NODE_ENV=test mocha -w test/specs.js",
-    "test:esm": "NODE_ENV=test mocha test/esm/unit.mjs test/esm/integration.js",
+    "testonly": "mocha test/specs.js",
+    "testonly-watch": "mocha -w test/specs.js",
+    "test:esm": "mocha test/esm/unit.mjs test/esm/integration.js",
     "test:all": "npm run testonly && npm run test:esm",
     "lint": "standard"
   },

register.mjs

@@ -2,6 +2,8 @@
 // Usage: node --import module-alias/register ./app.mjs
 
 import { register } from 'node:module'
+import { isAbsolute } from 'node:path'
+import { pathToFileURL } from 'node:url'
 
 // Check Node version for registerHooks support (22.15+)
 const [major, minor] = process.versions.node.split('.').map(Number)
@@ -18,6 +20,10 @@ if (hasRegisterHooks) {
     resolve (specifier, context, nextResolve) {
       const resolved = resolveAlias(specifier, context.parentURL)
       if (resolved) {
+        // If absolute path, convert to file URL
+        if (isAbsolute(resolved)) {
+          return { url: pathToFileURL(resolved).href, shortCircuit: true }
+        }
         return nextResolve(resolved, context)
       }
       return nextResolve(specifier, context)

test/esm/integration.js

@@ -3,6 +3,7 @@ var expect = require('chai').expect
 var execSync = require('child_process').execSync
 var path = require('path')
 var semver = require('semver')
+var pathToFileURL = require('url').pathToFileURL
 
 describe('ESM Integration', function () {
   // Skip all ESM tests on Node < 18.19.0
@@ -17,10 +18,10 @@ describe('ESM Integration', function () {
   describe('basic alias', function () {
     it('should resolve alias in ESM with --import flag', function () {
       var fixture = path.join(fixturesDir, 'basic')
-      var registerPath = path.join(moduleAliasRoot, 'register.mjs')
+      var registerUrl = pathToFileURL(path.join(moduleAliasRoot, 'register.mjs')).href
 
       var result = execSync(
-        'node --import ' + registerPath + ' app.mjs',
+        'node --import ' + registerUrl + ' app.mjs',
         { cwd: fixture, encoding: 'utf8' }
       )
 
@@ -31,10 +32,10 @@ describe('ESM Integration', function () {
   describe('moduleDirectories', function () {
     it('should resolve modules from custom directories', function () {
       var fixture = path.join(fixturesDir, 'module-dirs')
-      var registerPath = path.join(moduleAliasRoot, 'register.mjs')
+      var registerUrl = pathToFileURL(path.join(moduleAliasRoot, 'register.mjs')).href
 
       var result = execSync(
-        'node --import ' + registerPath + ' app.mjs',
+        'node --import ' + registerUrl + ' app.mjs',
         { cwd: fixture, encoding: 'utf8' }
       )
 

test/esm/unit.mjs

@@ -1,8 +1,8 @@
 /* eslint-env mocha */
 import chai from 'chai'
 import path from 'node:path'
-import { fileURLToPath } from 'node:url'
-import { isPathMatchesAlias, resolveAlias, reset, addAlias, addAliases } from '../../esm-loader.mjs'
+import { fileURLToPath, pathToFileURL } from 'node:url'
+import { isPathMatchesAlias, resolveAlias, reset, addAlias, addAliases, resolve } from '../../esm-loader.mjs'
 
 const { expect } = chai
 const __dirname = path.dirname(fileURLToPath(import.meta.url))
@@ -76,7 +76,7 @@ describe('ESM Loader', function () {
       })
 
       const result = resolveAlias('react-dom/server')
-      expect(result).to.equal('/path/to/server')
+      expect(result).to.equal(path.join('/path/to/server'))
     })
 
     it('should handle multiple aliases', function () {
@@ -85,8 +85,44 @@ describe('ESM Loader', function () {
         '@utils': '/path/to/utils'
       })
 
-      expect(resolveAlias('@lib/foo')).to.equal('/path/to/lib/foo')
-      expect(resolveAlias('@utils/bar')).to.equal('/path/to/utils/bar')
+      expect(resolveAlias('@lib/foo')).to.equal(path.join('/path/to/lib', 'foo'))
+      expect(resolveAlias('@utils/bar')).to.equal(path.join('/path/to/utils', 'bar'))
+    })
+  })
+
+  describe('resolve hook', function () {
+    afterEach(function () {
+      reset()
+    })
+
+    it('should convert absolute paths to file URLs', async function () {
+      // Platform-native absolute path (C:\... on Windows, /... on POSIX)
+      const targetPath = path.join(__dirname, '../src')
+      addAlias('@src', targetPath)
+
+      const result = await resolve('@src/foo', {}, () => {
+        throw new Error('nextResolve should not be called for absolute paths')
+      })
+      expect(result.url).to.equal(pathToFileURL(path.join(targetPath, 'foo')).href)
+      expect(result.shortCircuit).to.equal(true)
+    })
+
+    it('should pass non-absolute resolved specifiers to nextResolve', async function () {
+      addAlias('lodash-alias', 'lodash')
+
+      const result = await resolve('lodash-alias/fp', {}, (specifier) => {
+        return { url: 'resolved:' + specifier }
+      })
+      expect(result.url).to.equal('resolved:' + path.join('lodash', 'fp'))
+    })
+
+    it('should pass non-aliased specifiers through unchanged', async function () {
+      addAlias('@src', '/some/path')
+
+      const result = await resolve('other-module', {}, (specifier) => {
+        return { url: 'resolved:' + specifier }
+      })
+      expect(result.url).to.equal('resolved:other-module')
     })
   })
 })