How to declare a function type in typescript

Question

I am working with typescript and passing a function into another function.

If I have a function that is passed into another function in typescript how should I write the type?

I have tried successHandler: function but this doesn't appear to work.

export function useSubscription(address: string, successHandler: function) {
   successHandler(address)
}

Answer 1

Declare a type with a function signature and pass it around as the type:

type SuccessHandler = (address: string) => string;

function useSubscription(address: string, successHandler: SuccessHandler) {
    successHandler(address)
}

Answer 2

You can declare it like this:

export function useSubscription(address: string, successHandler: (string) => void)) {
   successHandler(address)
}

The change is with the capital F for Function.