Copydotnet.d.ts to wwwroot on build/publish.

src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets

@@ -98,6 +98,7 @@ Copyright (c) .NET Foundation. All rights reserved.
     <!-- Static Web Assets -->
     <ResolveStaticWebAssetsInputsDependsOn>
       $(ResolveStaticWebAssetsInputsDependsOn);
+      _EnsureDotnetTypeScriptDefinitions;
       _AddWasmStaticWebAssets;
     </ResolveStaticWebAssetsInputsDependsOn>
     <ResolvePublishStaticWebAssetsDependsOn Condition="'$(WasmBuildingForNestedPublish)' != 'true'">
@@ -856,4 +857,21 @@ Copyright (c) .NET Foundation. All rights reserved.
       <WasmAssembliesToBundle Include="@(ResolvedFileToPublish)" Exclude="@(_Exclude)" Condition="%(Extension) == '.dll'" />
     </ItemGroup>
   </Target>
+
+  <!-- Ensure dotnet.d.ts is available in wwwroot for TypeScript development experience -->
+  <Target Name="_EnsureDotnetTypeScriptDefinitions">
+    <PropertyGroup>
+      <_RuntimePackDir>$(MicrosoftNetCoreAppRuntimePackDir)</_RuntimePackDir>
+      <_RuntimePackDir Condition="'$(_RuntimePackDir)' == ''">%(ResolvedRuntimePack.PackageDirectory)</_RuntimePackDir>
+      <_RuntimePackNativeDir>$([MSBuild]::NormalizeDirectory($(_RuntimePackDir), 'runtimes', 'browser-wasm', 'native'))</_RuntimePackNativeDir>
+      <_DotnetTypesSourcePath>$(_RuntimePackNativeDir)dotnet.d.ts</_DotnetTypesSourcePath>
+      <_DotnetTypesDestPath>$(MSBuildProjectDirectory)\wwwroot\dotnet.d.ts</_DotnetTypesDestPath>
+    </PropertyGroup>
+
+    <!-- Copy dotnet.d.ts if source exists. MSBuild will skip if files are identical. -->
+    <Copy SourceFiles="$(_DotnetTypesSourcePath)"
+          DestinationFiles="$(_DotnetTypesDestPath)"
+          Condition="Exists('$(_DotnetTypesSourcePath)')"
+          SkipUnchangedFiles="true" />
+  </Target>
 </Project>

src/mono/wasm/Wasm.Build.Tests/Templates/WasmTemplateTests.cs

@@ -317,5 +317,27 @@ public async Task LibraryModeBuild(bool useWasmSdk)
             }
 
         }
+
+        [Theory]
+        [InlineData(Configuration.Debug)]
+        [InlineData(Configuration.Release)]
+        public void TypeScriptDefinitionsCopiedToWwwroot(Configuration config)
+        {
+            ProjectInfo info = CreateWasmTemplateProject(Template.WasmBrowser, config, aot: false, "tsdefs");
+
+            string projectDirectory = Path.GetDirectoryName(info.ProjectFilePath)!;
+            string dotnetDtsPath = Path.Combine(projectDirectory, "wwwroot", "dotnet.d.ts");
+
+            Assert.True(File.Exists(dotnetDtsPath), $"dotnet.d.ts should exist at {dotnetDtsPath} after creation");
+
+            // Remove the file to test that MSBuild target recreates it
+            File.Delete(dotnetDtsPath);
+            Assert.False(File.Exists(dotnetDtsPath), $"dotnet.d.ts should be deleted before the build");
+
+            // Build to trigger the _CopyDotnetTypeScriptDefinitions target
+            BuildProject(info, config, new BuildOptions());
+
+            Assert.True(File.Exists(dotnetDtsPath), $"dotnet.d.ts should be recreated at {dotnetDtsPath} after the build");
+        }
     }
 }