Empty tuples

In TypeScript 3, we can now define an empty tuple type. Let's have a little play with this in the TypeScript playground:

1. Let's create the following type alias for an empty tuple:

  type Empty = [];

1. Let's declare a variable of this type and assign it to an empty array:

  const empty: Empty = [];

1. Now, let's try to declare a variable of this type and assign it to a non-empty array:

  const notEmpty: Empty = ["Billy"];

As expected, we get a compilation error:

Why is an empty tuple type useful, though? On its own, it perhaps is not that useful, but it can be used as part of a union type, which we'll cover in detail later in the book. As a quick example for now, we can create a type for no more than three scores, where no scores is also acceptable:

type Scores = [] | [number] | [number, number] | [number, number, number]

const benScores: Scores = [];
const samScores: Scores = [55];
const bobScores: Scores = [95, 75];
const jayneScores: Scores = [65, 50, 70];
const sarahScores: Scores = [95, 50, 75, 75];

All the scores are valid except Sarah's, because four scores aren't allowed in the Scores type.