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

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

@@ -191,6 +191,7 @@ Copyright (c) .NET Foundation. All rights reserved.
       <_WasmCopyOutputSymbolsToOutputDirectory Condition="'$(_WasmCopyOutputSymbolsToOutputDirectory)'==''">true</_WasmCopyOutputSymbolsToOutputDirectory>
       <_WasmEnableThreads>$(WasmEnableThreads)</_WasmEnableThreads>
       <_WasmEnableThreads Condition="'$(_WasmEnableThreads)' == ''">false</_WasmEnableThreads>
+      <WasmEmitTypeScriptDefinitions Condition="'$(WasmEmitTypeScriptDefinitions)' == ''">false</WasmEmitTypeScriptDefinitions>
 
       <_WasmEnableWebcil>$(WasmEnableWebcil)</_WasmEnableWebcil>
       <_WasmEnableWebcil Condition="'$(_TargetingNET80OrLater)' != 'true'">false</_WasmEnableWebcil>
@@ -856,4 +857,23 @@ 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 when enabled -->
+  <Target Name="_EnsureDotnetTypeScriptDefinitions" 
+          AfterTargets="Restore" 
+          Condition="'$(WasmEmitTypeScriptDefinitions)' == 'true'">
+    <PropertyGroup>
+      <_RuntimePackDir>$(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 +317,26 @@ public async Task LibraryModeBuild(bool useWasmSdk)
             }
 
         }
+
+        [Theory]
+        [InlineData(Configuration.Debug)]
+        [InlineData(Configuration.Release)]
+        public void TypeScriptDefinitionsCopiedToWwwrootOnBuild(Configuration config)
+        {
+            string emitTypeScriptDts = "<WasmEmitTypeScriptDefinitions>true</WasmEmitTypeScriptDefinitions>";
+            ProjectInfo info = CreateWasmTemplateProject(Template.WasmBrowser, config, aot: false, "tsdefs", extraProperties: emitTypeScriptDts);
+
+            string projectDirectory = Path.GetDirectoryName(info.ProjectFilePath)!;
+            string dotnetDtsWwwrootPath = Path.Combine(projectDirectory, "wwwroot", "dotnet.d.ts");
+
+            // Verify dotnet.d.ts is not in wwwroot after creation
+            Assert.False(File.Exists(dotnetDtsWwwrootPath), $"dotnet.d.ts should not exist at {dotnetDtsWwwrootPath} after creation when WasmEmitTypeScriptDefinitions is used");
+
+            // Build to trigger the _EnsureDotnetTypeScriptDefinitions target on restore
+            BuildProject(info, config, new BuildOptions(WasmEmitTypeScriptDefinitions: true));
+
+            // Verify dotnet.d.ts is created in the project's wwwroot directory after build
+            Assert.True(File.Exists(dotnetDtsWwwrootPath), $"dotnet.d.ts should be created at {dotnetDtsWwwrootPath} after the build with WasmEmitTypeScriptDefinitions=true");
+        }
     }
 }

src/mono/wasm/build/WasmApp.Common.targets

@@ -93,6 +93,8 @@
                                               - AppBundle/_content contains web files from nuget packages (css, js, etc)
       - $(WasmStripILAfterAOT)              - Set to true to enable trimming away AOT compiled methods body (IL code)
                                               Defaults to true.
+      - $(WasmEmitTypeScriptDefinitions)    - Controls whether TypeScript definitions (dotnet.d.ts) should be copied to the output. 
+                                              Defaults to false.
 
       Public items:
       - @(WasmExtraFilesToDeploy) - Files to copy to $(WasmAppDir).
@@ -189,6 +191,7 @@
     <WasmStripILAfterAOT Condition="'$(WasmStripILAfterAOT)' == ''">true</WasmStripILAfterAOT>
     <WasmRuntimeAssetsLocation Condition="'$(WasmRuntimeAssetsLocation)' == ''">_framework</WasmRuntimeAssetsLocation>
     <MetricsSupport Condition="'$(MetricsSupport)' == ''">false</MetricsSupport>
+    <WasmEmitTypeScriptDefinitions Condition="'$(WasmEmitTypeScriptDefinitions)' == ''">false</WasmEmitTypeScriptDefinitions>
 
     <!-- semicolon is a msbuild property separator. It is also the path separator on windows.
          So, we need to escape it here, so the paths don't get split up when converting