Can you explain this odd ES6 arrow function in TypeScript?

Question

using TypeScript:

public subscribe:(subscribeFunction:(state)=>void)=>()=>void;

It's a subscribe method that gets function as an argument, of type function, and that function given, will receive a state argument when called, that argumented function will not return anything (i.e.: void) ... and I am lost on the last ()=>()=>void.

Do I understand this right?

Answer 1

It's for a public property called subscribe that has a type of (subscribeFunction: (state) => void) => () => void;:

// scope
public
// name
subscribe:
// type (function)
    // parameters
    (
        // parameter name
        subscribeFunction:
        // parameter type (function)
        (state) => void
    ) => 
    // return type (function)
    () => void;

Here's an example that compiles:

class MyClass {
    public subscribe: (subscribeFunction: (state) => void) => () => void;
}

let myInstance = new MyClass();

myInstance.subscribe = (subscribeFunction: (state) => void) => {
    console.log("statements might go here");

    return () => {
        subscribeFunction(1 /* state */);
        console.log("nothing returned by this inner function");
    };
};

// Example use

// outputs "statements might go here"
let innerFunction = myInstance.subscribe((state) => console.log(state));
// outputs 1 and "nothing returned by this inner function"
innerFunction();