Is there a way to determine if a tuple is of infinite or finite length?

Is there a way to determine if a tuple is finite or infinite?

I've been working on a solution, but it doesn't cover all cases:

type IsFinite<T extends any[], Finite = true, Infinite = false> = T extends []
  ? Finite
  : T extends (infer Element)[]
  ? Element[] extends T
    ? Infinite
    : T extends [any, ...infer Rest]
    ? IsFinite<Rest, Finite, Infinite>
    : never
  : never

 type Foo1 = IsFinite<[string]>                          // true
 type Foo2 = IsFinite<[]>                                // true
 type Foo3 = IsFinite<string[]>                          // false
 type Foo4 = IsFinite<[arg1: string, ...args: string[]]> // false

 type Foo5 = IsFinite<[string, ...[string, number], ...string[], string]> // never <- doesn't handle this case

Try it out

typescript

Answer №1

When referring to an "infinite" array/tuple, it denotes an array type with no specified maximum length in TypeScript. In this case, the length property of the array type would be of type number, as opposed to a "finite" array/tuple where the length property is a numeric literal type or union of such literals:

type Length<T extends any[]> = T['length'];

type L1 = Length<[string]>                          // 1
type L2 = Length<[]>                                // 0
type L3 = Length<string[]>                          // number
type L4 = Length<[arg1: string, ...args: string[]]> // number
type L5 = Length<[string, ...[string, number], ...string[], string]> // number
type L6 = Length<[string, string?]> //  1 | 2

To define IsFinite<T>, you can use a conditional type that checks the type of the length property:

type IsFinite<T extends any[], F = true, I = false> =
    number extends T['length'] ? I : F

type Foo1 = IsFinite<[string]>                          // true
type Foo2 = IsFinite<[]>                                // true
type Foo3 = IsFinite<string[]>                          // false
type Foo4 = IsFinite<[arg1: string, ...args: string[]]> // false
type Foo5 = IsFinite<[string, ...[string, number], ...string[], string]> // false
type Foo6 = IsFinite<[string, string?]> //  true

Link to Playground for code testing

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