I just read that there is also a type of javascript called server-side javascript, before it i was thinking that javascript is used only at client side....so my question is what is server-side javascript ...and for a front-end developer , is it necessary to learn both types of javascript?
Server side javascript is somehow like java/C++/.Net/ruby/php... It is just an approach of programming server code. If you work on the front-end without any back-end work to do, it is not necessarily to learn server-side javascript. Server side javascript you mentioned is probably nodejs. Imagine that you can write a server with js instead of java or php.