[Blazor] Fix JSInitializer URL computation for base URLs with query parameters

src/Components/Web.JS/src/JSInitializers/JSInitializers.ts

@@ -49,10 +49,9 @@ export class JSInitializer {
     await Promise.all(initializerFiles.map(f => importAndInvokeInitializer(this, f)));
 
     function adjustPath(path: string): string {
-      // This is the same we do in JS interop with the import callback
-      const base = document.baseURI;
-      path = base.endsWith('/') ? `${base}${path}` : `${base}/${path}`;
-      return path;
+      // Use URL constructor to properly resolve relative paths, avoiding issues with query parameters
+      // This is the same mechanism as for import dotnet.js in MonoPlatform.ts
+      return new URL(path, document.baseURI).toString();
     }
 
     async function importAndInvokeInitializer(jsInitializer: JSInitializer, asset: JSAsset): Promise<void> {

src/Components/Web.JS/test/JSInitializers.test.ts

@@ -0,0 +1,98 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+describe('JSInitializer URL handling', () => {
+  // Mock document.baseURI for testing
+  let originalBaseURI: string;
+
+  beforeEach(() => {
+    originalBaseURI = document.baseURI;
+  });
+
+  afterEach(() => {
+    // Reset document.baseURI by setting a new base element
+    const existingBase = document.querySelector('base');
+    if (existingBase) {
+      existingBase.remove();
+    }
+    const base = document.createElement('base');
+    base.href = originalBaseURI;
+    document.head.insertBefore(base, document.head.firstChild);
+  });
+
+  function setDocumentBase(baseUri: string): void {
+    // Remove existing base elements
+    const existingBases = document.querySelectorAll('base');
+    existingBases.forEach(base => base.remove());
+
+    // Add new base element
+    const base = document.createElement('base');
+    base.href = baseUri;
+    document.head.insertBefore(base, document.head.firstChild);
+  }
+
+  // Test the correct URL resolution approach that the fix implements
+  function correctUrlResolution(path: string): string {
+    return new URL(path, document.baseURI).toString();
+  }
+
+  // Test the problematic string concatenation approach that was fixed
+  function problematicStringConcatenation(path: string): string {
+    const base = document.baseURI;
+    return base.endsWith('/') ? `${base}${path}` : `${base}/${path}`;
+  }
+
+  test('URL constructor vs string concatenation with query parameters', () => {
+    setDocumentBase('http://domain?a=x');
+
+    const correctResult = correctUrlResolution('_content/Package/file.js');
+    const problematicResult = problematicStringConcatenation('_content/Package/file.js');
+
+    // The URL constructor produces a valid URL
+    expect(correctResult).toBe('http://domain/_content/Package/file.js');
+
+    // String concatenation produces malformed URL with query in wrong place  
+    expect(problematicResult).toBe('http://domain/?a=x/_content/Package/file.js');
+
+    // Verify they are different
+    expect(correctResult).not.toBe(problematicResult);
+  });
+
+  test('URL constructor handles hash correctly', () => {
+    setDocumentBase('http://domain#section');
+    const result = correctUrlResolution('_content/Package/file.js');
+
+    // Hash should be dropped when resolving relative URLs
+    expect(result).toBe('http://domain/_content/Package/file.js');
+  });
+
+  test('URL constructor handles trailing slash correctly', () => {
+    setDocumentBase('http://domain/');
+    const result = correctUrlResolution('_content/Package/file.js');
+
+    expect(result).toBe('http://domain/_content/Package/file.js');
+  });
+
+  test('URL constructor handles no trailing slash correctly', () => {
+    setDocumentBase('http://domain');
+    const result = correctUrlResolution('_content/Package/file.js');
+
+    expect(result).toBe('http://domain/_content/Package/file.js');
+  });
+
+  test('URL constructor resolves from subdirectory correctly', () => {
+    setDocumentBase('http://domain/subdir/');
+    const result = correctUrlResolution('_content/Package/file.js');
+
+    // From a subdirectory base, the relative path resolves relative to that directory
+    expect(result).toBe('http://domain/subdir/_content/Package/file.js');
+  });
+
+  test('URL constructor handles complex base with query and hash', () => {
+    setDocumentBase('http://domain/app/?debug=true#section');
+    const result = correctUrlResolution('_content/Package/file.js');
+
+    // Query and hash are handled properly - query is dropped, hash is dropped
+    expect(result).toBe('http://domain/app/_content/Package/file.js');
+  });
+});