Directory.Build.targets

<Project>
  <Target Name="NpmInstall" BeforeTargets="BeforeBuild" Inputs="@(PackageJsons)" Outputs="$(NpmInstallStampFile)">
    <Message Importance="high" Text="Installing packages from %(PackageJsons.Identity)"/>
    <Exec Command="npm install" WorkingDirectory="%(PackageJsons.RootDir)%(PackageJsons.Directory)"/>
    <Touch Files="$(NpmInstallStampFile)" AlwaysCreate="true" />
  </Target>
</Project>