[braythwayt]

  > I don't think programming languages implement decorators in order to facilitate
  > type checking. They implement decorators because they are a nice generic way to
  > reuse code in many functions / classes.

Well, in JavaScript, it’s particularly easy to write decorators as plain old JavaScript functions. Lots of libraries provide implementations that provide memoization or parameter checking on both functions and methods.

So in the short term, decorators serve only to work around the fact that ES-6 classes actually make this more difficult than ES-5 idioms.

But in the long term, a decorator can be extended to deal with static analysis, while functions provid eno such capability, so we’d be back to having “magic comments,” or inventing a new kind of pragma.

So, I’d say that the motivation is to lay the groundwork for things involving static analysis or manipulation of programs. Like type checking, or even manipulation of the AST.