Why FileExtensionContentTypeProvider no work with .min.js extension? - asp.net-core

Question

I have some css&js files in my project and I used the BuildBundlerMinifier NuGet package to minify and obfuscate them.
For example, the app.js will minify and obfuscate into app.min.js in the same directory.
Now I want the user can access the app.min.js but can't access the app.js.
I do this for I don't want anybody else to access the source code of my js.
Although someone still can get its source code from the app.min.js while I don't want them to get it easily.
I tried to use FileExtensionContentTypeProvider in Configure of startup.cs to achieve this:
var provider = new Microsoft.AspNetCore.StaticFiles.FileExtensionContentTypeProvider();
provider.Mappings.Remove(".js");
provider.Mappings.Add(new KeyValuePair<string, string>(".min.js", "application/javascript"));
app.UseStaticFiles(new StaticFileOptions()
{
ContentTypeProvider=provider
});
However, after it runs I can access neither app.js nor app.min.js.
What's wrong with my code?
Thank you.

Answer 1

The FileExtensionContentTypeProvider is only meant to provide a mapping from file extension to the correct MIME type. In order to retrieve the file extension from a file name, it will do the following:
private static string? GetExtension(string path)
{
int index = path.LastIndexOf('.');
if (index < 0)
return null;
return path.Substring(index);
}
It will take the very last part of the extension. So with app.min.js, the reported file extension will still be .js and not .min.js, and as such the mapping for .js will be required.
Modifying the MIME type mapping in order to disallow certain file extensions is probably not the best strategy. It would be better to modify the underlying file provider itself to handle that.
Alternatively, if you want to prevent access to non-minified JavaScript files, you could also split the middleware to conditionally prevent serving static files for any request to a path that ends with .js that is not a .min.js:
app.UseWhen(ctx => !ctx.Request.Path.HasValue
|| !ctx.Request.Path.Value.EndsWith(".js")
|| ctx.Request.Path.Value.EndsWith(".min.js"), app2 =>
{
app2.UseStaticFiles();
});