Typescript Interface make optional

Question

I have an interface like this:

export interface IDefaultAction extends Object {
  type: string
  (dispatch: Dispatch<IStateObject>, getState: () => IStateObject, extraArgument: any): any;
}

is there any way I can make the second line in the interface optional? (dispatch: Dispatch<IStateObject>, getState: () => IStateObject, extraArgument: any): any;

and if so, how?

and also if possible please explain or point me to the right documentation which explains what does this interface mean:

interface IA {
  ():any;
}

I just can't figure out this syntax

():something;

Thanks!

edit:

I am trying to extend this:

export type ThunkAction<R, S, E> = (dispatch: Dispatch<S>, getState: () => S,
                                    extraArgument: E) => R;

in my own interface:

export interface IDefaultAction {
  type: string;
}

but optionally, so the only thing I could think of, is to modify the original(ThunkAction) and make all inside it optional, but I don't see how.

Shaun Luttin · Accepted Answer · 2017-08-27 14:40:28Z

please explain or point me to the right documentation which explains what does this interface mean:

The IA interface is a function interface. It defines a "function type".

interface IA {
    (): any;
}

const exampleImplementation: IA = () => "";

The (): any defines the function type's signature, which includes the function's parameter list and return type. The function type IA takes no parameters and returns an any.

is there any way I can make the second line in the interface optional?

The second line is a function signature, which means that the interface defines a function type. Its function signature takes two parameters and returns an any.

export interface IDefaultAction extends Object {
  type: string;
  (
    dispatch: Dispatch<IStateObject>,                  // paramater 1
    getState: () => IStateObject, extraArgument: any   // parameter 2
  ) : any;                                             // return type
}

While interfaces support optional properties, interfaces do not support optional function signatures.

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