Mark Trostler has a great presentation on creating testable JavaScript.

• Create all of the objects the system will ever need at the start (composition root) by building a tree of dependencies.
  • Create general objects at application start and user/session specific objects at the request start.
    • Use an IOC/DI framework that is configured once at the start to constructor inject the dependencies.
• Cross-cutting concerns - Concerns that affect many areas of your application. They cut across your application and do not align with modules.
  • Keep orthogonal responsibilities separate.
  • Examples: Logging, validation, auditing, caching, security, profiling, exception handling, retry logic, authentication, authorization, encryption, etc.
  • Use decorator pattern to create wrappers around your business objects that handle the cross-cutting concerns.
  • Use your IOC container to wire up decorator chains.
• Run-time (short-lived) dependancies - Can’t pass into constructor because we don’t know what they will need until run-time (user has to pick).
  • Pass run-time values into a factory pattern to get dependencies.
  • Factory implementation is created at composition root and injected as a dependency via the constructor.
• Program and test to interfaces.
  • Interfaces are the API.
  • Don’t worry about hiding via closures (revealing module pattern) just use the interfaces.
  • Create many small interfaces that have a single responsibility.

Presentation Link