In short: the C# script has to be compiled before the JS script for this to work. To achieve this you have to move the C# script into one of the following directories: Plugins, Standard Assets or Pro Standard Assets.
It also works the other way around – if you want to call JS from C#, move the JS into one of the folders.
It does not work both ways at once however.
More info about script compilation order can be found in the Unity docs here…