What does the following mean as a type: () => void = null

Question

I'm looking at these two function declarations:

function f1(chainFn: (fn: Function) => void = null) {

}

function f2(): (...args: any[]) => (cls: any) => any {

}

I don't really understand what the following parts define:

// function that accepts `fn` of Function type by returns what?
(fn: Function) => void = null

// function that accepts an array of arguments and returns a function that returns a result of `any` type?
(...args: any[]) => (cls: any) => any

Can anyone please elaborate and possible provide examples of concrete implementations?

Answer 1

() => void

Is the type of a function which takes no arguments and does not return anything.

() => void = null

Is not a type at all.

function f(g: () => void = null) { ... }

Is a function f which takes another function, g of type () => void, as an argument and provides a default value of null for that argument if it is not provided. This also renders the argument optional.

I would like to add that using null as the default value of an optional argument is a terrible practice because JavaScript passes undefined not null for unspecified arguments and there's no reason to change this behaviour as doing so is surprising. In such a case it is preferable to either write

// g is optional and will be undefined if not provided
f(g?: () => void) { ... }

or to write

// g is optional and will be a no-op if not provided.
f(g: () => void = () => {}) { ... }

Answer 2

The first function f1 accepts an argument chainFn and it is a function that as a parameter accepts a function fn and does not return anything => void, also this parameter chainFn is optional = null with runtime default value of undefined.

The second function f2 does not accept any arguments and returns a function. That function accepts open parameters list of any type ...args:any[] (you can call it using comma separated arguments var r = f2(); r(1,2,3,4);) and it returns a function that accepts any type as parameters and returns something.