Fixed Length Tuples

In TypeScript 2.6 and earlier, [number, string, string] was considered a subtype of [number, string].This was motivated by TypeScript’s structural nature; the first and second elements of a [number, string, string] are respectively subtypes of the first and second elements of [number, string].However, after examining real world usage of tuples, we noticed that most situations in which this was permitted was typically undesirable.

In TypeScript 2.7, tuples of different arities are no longer assignable to each other.Thanks to a pull request from Tycho Grouwstra, tuple types now encode their arity into the type of their respective length property.This is accomplished by leveraging numeric literal types, which now allow tuples to be distinct from tuples of different arities.

Conceptually, you might consider the type [number, string] to be equivalent to the following declaration of NumStrTuple:

interface NumStrTuple extends Array<number | string> {
    0: number;
    1: string;
    length: 2; // using the numeric literal type '2'
}

Note that this is a breaking change for some code.If you need to resort to the original behavior in which tuples only enforce a minimum length, you can use a similar declaration that does not explicitly define a length property, falling back to number.

interface MinimumNumStrTuple extends Array<number | string> {
    0: number;
    1: string;
}

Note that this does not imply tuples represent immutable arrays, but it is an implied convention.