You can write in other languages beyond JavaScript or TypeScript and generate WebAssembly. This means that say someone wrote a nice library or utility in C# you can compile that C# down to WebAssembly and use that library or utility in JavaScript or TypeScript (or anything else that can access WebAssembly).
This is analogous perhaps to C# and F# within .NET currently. C# and F# have the same BCL (base common layer) so you can use C# code in F# and vice versa. WebAssembly is like that and much more.
WebAssembly would be analogous to CLI (Common Language Infrastructure) in .NET land, which includes CIL (Common Intermediate Language - bytecode) and CLR (Common Language Runtime - VM).
.NET BCL is the Base Class Library. Outside of a few classes in it that are fundamental to the runtime (e.g. Object, String, Array), it's not actually required for cross-language interop on .NET platforms. E.g. if you have two different C compilers both outputting CIL, they can interop just fine without any objects in the picture. WebAssembly interop is really more like that, and doesn't have the high-level object model layer that .NET also has (and which is used in practice).
This is analogous perhaps to C# and F# within .NET currently. C# and F# have the same BCL (base common layer) so you can use C# code in F# and vice versa. WebAssembly is like that and much more.