fix: handle Windows backslash separators in isUnderMountPoint

lib/router.js

@@ -20,14 +20,24 @@ function getBaseName(normalizedPath) {
   return pathPosix.basename(normalizedPath);
 }
 
+// normalizeVFSPath uses pathPosix.normalize for /-prefixed paths and
+// path.normalize for native paths (which produces \ on Windows).  Both
+// styles can reach isUnderMountPoint, so we accept either separator.
+function isPathSeparator(char) {
+  return char === '/' || char === '\\';
+}
+
 function isUnderMountPoint(normalizedPath, mountPoint) {
   if (normalizedPath === mountPoint) {
     return true;
   }
-  if (mountPoint === '/') {
-    return normalizedPath.startsWith('/');
+  if (!normalizedPath.startsWith(mountPoint)) {
+    return false;
   }
-  return normalizedPath.startsWith(mountPoint + '/');
+  // The path starts with the mount point — accept it only when the very next
+  // character is a path separator (e.g. /app  →  /app/file) or the mount
+  // point itself already ends with one (e.g. C:\ or /).
+  return isPathSeparator(normalizedPath[mountPoint.length]) || isPathSeparator(mountPoint[mountPoint.length - 1]);
 }
 
 function getRelativePath(normalizedPath, mountPoint) {

test/mount.test.js

@@ -95,6 +95,76 @@ describe('VirtualFileSystem - mount/unmount', () => {
   });
 });
 
+describe('VirtualFileSystem - backslash path mount', () => {
+  let vfs;
+
+  beforeEach(() => {
+    vfs = create({ moduleHooks: false });
+  });
+
+  afterEach(() => {
+    if (vfs.mounted) {
+      vfs.unmount();
+    }
+  });
+
+  it('shouldHandle accepts backslash paths under mount point', () => {
+    vfs.mount('C:\\app');
+    assert.strictEqual(vfs.shouldHandle('C:\\app\\file.txt'), true);
+    assert.strictEqual(vfs.shouldHandle('C:\\app'), true);
+  });
+
+  it('shouldHandle rejects backslash paths outside mount point', () => {
+    vfs.mount('C:\\app');
+    assert.strictEqual(vfs.shouldHandle('C:\\other\\file.txt'), false);
+  });
+
+  it('shouldHandle rejects backslash paths that are a prefix but not a child', () => {
+    vfs.mount('C:\\app');
+    assert.strictEqual(vfs.shouldHandle('C:\\application\\file.txt'), false);
+  });
+
+  it('shouldHandle works with a drive root mount', () => {
+    vfs.writeFileSync('/data.json', '{}');
+    vfs.mount('C:\\');
+    assert.strictEqual(vfs.shouldHandle('C:\\data.json'), true);
+    assert.strictEqual(vfs.shouldHandle('C:\\deep\\nested\\path'), true);
+  });
+
+  it('mountPoint preserves backslashes', () => {
+    vfs.mount('C:\\app');
+    assert.strictEqual(vfs.mountPoint, 'C:\\app');
+  });
+});
+
+describe('VirtualFileSystem - Windows path I/O', { skip: process.platform !== 'win32' }, () => {
+  let vfs;
+
+  beforeEach(() => {
+    vfs = create({ moduleHooks: false });
+  });
+
+  afterEach(() => {
+    if (vfs.mounted) {
+      vfs.unmount();
+    }
+  });
+
+  it('readFileSync works through a Windows mount point', () => {
+    vfs.writeFileSync('/file.txt', 'windows mount content');
+    vfs.mount('C:\\mnt');
+    const content = vfs.readFileSync('C:\\mnt\\file.txt', 'utf8');
+    assert.strictEqual(content, 'windows mount content');
+  });
+
+  it('throws ENOENT for paths outside Windows mount point', () => {
+    vfs.mount('C:\\app');
+    assert.throws(() => vfs.readFileSync('C:\\outside\\file.txt'), {
+      code: 'ENOENT',
+    });
+  });
+});
+
 describe('VirtualFileSystem - overlay mode', () => {
   let vfs;
 

test/router.test.js

@@ -0,0 +1,101 @@
+'use strict';
+
+const { describe, it } = require('node:test');
+const assert = require('node:assert/strict');
+const {
+  isUnderMountPoint,
+  getRelativePath,
+  splitPath,
+  getParentPath,
+  getBaseName,
+} = require('../lib/router.js');
+
+describe('isUnderMountPoint', () => {
+  it('returns true when path equals mount point', () => {
+    assert.strictEqual(isUnderMountPoint('/app', '/app'), true);
+  });
+
+  it('returns true for path under posix mount point', () => {
+    assert.strictEqual(isUnderMountPoint('/app/file.txt', '/app'), true);
+  });
+
+  it('returns false for path not under mount point', () => {
+    assert.strictEqual(isUnderMountPoint('/other/file.txt', '/app'), false);
+  });
+
+  it('returns false for path that is a prefix but not a child', () => {
+    assert.strictEqual(isUnderMountPoint('/application/file.txt', '/app'), false);
+  });
+
+  it('returns true for any path when mount point is /', () => {
+    assert.strictEqual(isUnderMountPoint('/file.txt', '/'), true);
+    assert.strictEqual(isUnderMountPoint('/deep/nested/path', '/'), true);
+  });
+
+  // Windows-style paths (backslash separators)
+
+  it('returns true when Windows path equals mount point', () => {
+    assert.strictEqual(isUnderMountPoint('C:\\foo\\mount', 'C:\\foo\\mount'), true);
+  });
+
+  it('returns true for Windows path under mount point', () => {
+    assert.strictEqual(isUnderMountPoint('C:\\foo\\mount\\file.js', 'C:\\foo\\mount'), true);
+  });
+
+  it('returns false for Windows path not under mount point', () => {
+    assert.strictEqual(isUnderMountPoint('C:\\bar\\file.js', 'C:\\foo\\mount'), false);
+  });
+
+  it('returns false for Windows path that is a prefix but not a child', () => {
+    assert.strictEqual(isUnderMountPoint('C:\\foo\\mountextra\\file.js', 'C:\\foo\\mount'), false);
+  });
+
+  it('returns true for paths under a Windows drive root mount', () => {
+    assert.strictEqual(isUnderMountPoint('C:\\file.txt', 'C:\\'), true);
+    assert.strictEqual(isUnderMountPoint('C:\\deep\\nested\\path', 'C:\\'), true);
+  });
+
+  it('returns true for nested path under Windows mount point', () => {
+    assert.strictEqual(isUnderMountPoint('C:\\foo\\mount\\sub\\dir\\file.js', 'C:\\foo\\mount'), true);
+  });
+});
+
+describe('getRelativePath', () => {
+  it('returns / when path equals mount point', () => {
+    assert.strictEqual(getRelativePath('/app', '/app'), '/');
+  });
+
+  it('returns full path when mount point is /', () => {
+    assert.strictEqual(getRelativePath('/file.txt', '/'), '/file.txt');
+  });
+
+  it('strips the mount point prefix', () => {
+    assert.strictEqual(getRelativePath('/app/file.txt', '/app'), '/file.txt');
+  });
+});
+
+describe('splitPath', () => {
+  it('returns empty array for root', () => {
+    assert.deepStrictEqual(splitPath('/'), []);
+  });
+
+  it('splits a path into segments', () => {
+    assert.deepStrictEqual(splitPath('/a/b/c'), ['a', 'b', 'c']);
+  });
+});
+
+describe('getParentPath', () => {
+  it('returns null for root', () => {
+    assert.strictEqual(getParentPath('/'), null);
+  });
+
+  it('returns parent directory', () => {
+    assert.strictEqual(getParentPath('/a/b'), '/a');
+  });
+});
+
+describe('getBaseName', () => {
+  it('returns base name', () => {
+    assert.strictEqual(getBaseName('/a/b/c.txt'), 'c.txt');
+  });
+});