Reusable type annotation for function declaration in Typescript? [duplicate]

Question

type Func = (foo:string) => void

// function expression
const myFunctionExpression:Func = function(foo) {
  console.log(foo)
}

In the Typescript snippet above, I am using type alias to annotate the variable in a function expression.

The type alias:

type Func = (foo:string) => void

is reusable in another function expression to reduce repetition.

My question is: Is there a way to reuse this type alias to annotate a function declaration ?

// function declaration
function myFunctionDeclaration(foo:string):void {
  console.log(foo)
}

After some search online, I cannot seem to find such syntax, what am I missing?

Thanks

update:

At the time of this writing there is a ticket on github requesting for this feature: Suggestion: Type annotations and interfaces for function declarations #22063 (thanks to comment from @jcalz)

Answer 1

At time of writing (TypeScript 3.4), there is not a way to apply a type to a function declaration.

Answer 2

You can use the typescript utility types available from version 3.1 to achieve this.

1. Parameters<Type> : https://www.typescriptlang.org/docs/handbook/utility-types.html#parameterstype
2. ReturnType<Type> : https://www.typescriptlang.org/docs/handbook/utility-types.html#returntypetype

---

Example fitting the question:

Typescript Sandbox

type Func = (foo:string) => void

// function expression
const myFunctionExpression:Func = function(foo) {
  console.log(foo)
}
// function declaration
function myFunctionDeclaration(...[foo]: Parameters<Func>): ReturnType<Func> {
  console.log(foo)
}