The only way to prevent cheating (and unlocking freemium items is a kind of cheating) is to move all the relevant game logic to the server. Forget about obfuscation. When it's executed on the clients machine, it's under the control of the user. Obfuscation can slow the user down, but it can't stop someone who is determined to hack it. This doesn't just apply to javascript, it applies to every game client, no matter what programming language it is written in.
A general rule of designing cheat-proof multiplayer games is to never trust the client and do all calculations which are relevant for gameplay on the server.