Remember that JavaScript was some day Netscape LiveScript and it was already a client and server programming language. In fact, ASP Classic could be developed using JScript (basically, old JavaScript).
A language turns into a server-side language once it can be used to develop back-end code, and yes, JavaScript has been always a player in the server development.
Now with NodeJS, JavaScript has been turned into a possible and serious language of choice to develop backends.
