Typescript decorator for function, not method. Possible?

add a custom TypeScript decorator in a function

Not to a raw function. The main issue is dealing with hoisting of functions. Any attempt to wrap a function in another function breaks the hoist.

Support Targets

A Decorator is a special kind of declaration that can be attached to a class declaration, method, accessor, property, or parameter. Decorators use the form @expression, where expression must evaluate to a function that will be called at runtime with information about the decorated declaration.

Docs: https://www.typescriptlang.org/docs/handbook/decorators.html#decorators