Does VS Code support server-side code in ASPX files?

VSCode is built to support .NET Core apps. .NET Core does not support ASP.NET Web Forms (and they probably never will). As noted correctly it is evaluated as HTML instead now.

That said, I don't think the issue in particular is VSCode, but more the compiler that isn't implemented to support Web Forms. I guess they just left out support for the ASPX syntax out of VSCode for that reason.

There is now an extension in the Marketplace called HTML (C#) which is a port from a Sublime plugin. I found it by searching for aspx in the Extensions sidebar. Picture of Marketplace Extension Search

Here's the link to the Github repo if that suits you better: https://github.com/fireside21/vscode-cshtml

Once that was loaded up I had no more errors from the incorrect assumption that I was editing a .js file.