Using JavaScript and C# inside one script

In the 4 months I'm using Unity, I've only used JavaScript.

I believe I'd read that one can use JavaScript AND C# in the same script, but I cannot remember where.

If this is correct, what script should this be? (In the editor do I select Scripts->CreateNew->JavaScript OR C Sharp Script?)

What I have at the moment, is a JS in which I want to use a List<>.

The first line of the script is:

using System.Collections.Generic;

Inside the script, there are JS lines and C# lines (I use C# to create and manipulate the List).

When I hit save, I get the error: ....(scriptName.js (1,6): UCE0001: ';' expected. Insert a semicolon at the end.

What did I do wrong?

I believe I'd read that one can use JavaScript AND C# in the same script, but I cannot remember where.

No, you read that you can use them in the same project.

I've never heard of having both types of script in the same file, but you can have them in the same project, scene, object, as different files of course. I think it would be confusing a 'bad practice' to try to mix script languages in the same file, even if it could be done.