variadic-tuple-types.ts

// Variadic tuple elements

type Foo<T extends unknown[]> = [string, ...T, number];

type T1 = Foo<[boolean]>;  // [string, boolean, number]
type T2 = Foo<[number, number]>;  // [string, number, number, number]
type T3 = Foo<[]>;  // [string, number]

// Strongly typed tuple concatenation

function concat<T extends unknown[], U extends unknown[]>(t: [...T], u: [...U]): [...T, ...U] {
    return [...t, ...u];
}

const ns = [0, 1, 2, 3];  // number[]

const t1 = concat([1, 2], ['hello']);  // [number, number, string]
const t2 = concat([true], t1);  // [boolean, number, number, string]
const t3 = concat([true], ns);  // [boolean, ...number[]]