How do I define a TypeScript interface for an empty array?

Question

This interface describes my answers array (IActivityAnswer[]).

export interface IActivityAnswer {
  createdAt: string;
  id: string;
  questionId: string;
  score: number;
  summary: string;
  title: string;
  commentCount?: number;
}

Some users won't have any answers (so they will have an empty array). How do I define a type for empty array. I want to accomplish this without using type any.

Answer 1

As of TypeScript 3.0, you can use the empty tuple type, written [].

const empty: [] = [];

However, even without this feature, I don't understand why you would need to use any since you could write something like

const empty = <never[] & {length: 0}>[];

The empty tuple type [] should be preferred as it provides more robust checking and better error messages when misused.

Answer 2

You can specify IActivityAnswer[] and the type would be defined as the following:

An array containing IActivityAnswer objects, or an array not containing IActivityAnswer objects, i.e. an empty array.