Target to remove old typescript output files after msbuild. This will remove all javascript files found in the typescript output directory that were not generated by typescript.

target.xml

  <Target Name="TypeScriptCleanOldOutput" DependsOnTargets="PreComputeCompileTypeScript" AfterTargets="Clean;Build" Condition=" '$(TypeScriptOutDir)' != '' ">
    <!--
      Assumes the TypeScriptOutDir only contains generated code.

      If a TypeScript file is deleted from the project the old js file
      will remain in the output folder.

      Find JS files in the output folder that were not generated by
      TypeScript and delete them.
    -->
    <ConvertToAbsolutePath Paths="$(TypeScriptOutDir)">
      <Output TaskParameter="AbsolutePaths" PropertyName="TypeScriptOutDirAbsolute"/>
    </ConvertToAbsolutePath>
    <PropertyGroup>
      <TypeScriptOutDirAbsolute Condition="!HasTrailingSlash('$(TypeScriptOutDirAbsolute)')">$(TypeScriptOutDirAbsolute)\</TypeScriptOutDirAbsolute>
    </PropertyGroup>
    <ItemGroup>
      <OldGeneratedJavascript Include="$(TypeScriptOutDirAbsolute)**\*.js" Exclude="@(GeneratedJavascript)" />
      <OldGeneratedJavascript Include="$(TypeScriptOutDirAbsolute)**\*.js.map" Exclude="@(GeneratedJavascript)" />
    </ItemGroup>
    <Delete Files="@(OldGeneratedJavascript)" />
  </Target>

[System.IO.Path]::GetFullPath can error in VS2015 because the current working directory in msbuild was C:\Windows\System32. GetFullPath resolves relative to the working directory rather than the project directory. Using ConvertToAbsolutePath instead.