I’ve heard that the JavaScript for unity is different than the one used in web browsers, and Codeacademy seems to advertise a web browser use for it. If I learn it from Codeacademy will I be able to use it in Unity?
No; Unity doesn’t use web Javascript. It’s a custom language more accurately referred to as Unityscript. Learning Actionscript3 would be more transferrable to Unity, since the syntax is very similar to Unityscript.