Variadic tuples

Learn variadic tuple types.

What is a variadic tuple?

A variadic tuple is a variable-length tuple whose last element can be spread with the ... operator. It is written as [A, ...B[]]. This way the leading elements are fixed while the rest are flexible in count. It is especially used to extend function parameters type-safely.

type Headed<T> = [string, ...T[]];
const x: Headed<number> = ["title", 1, 2, 3];

Concatenation with tuple spread

To concatenate two tuples, the form [...A, ...B] is used, where A and B are tuple types. Unlike array concatenation, this preserves the element types. For example, merging [number, ...string[]] with [boolean] yields [number, ...string[], boolean].

type Concat<A extends unknown[], B extends unknown[]> = [...A, ...B];

Exercise Write a headed tuple

Define a generic type named Baslikli: <T> = [string, ...T[]]. The y variable should be of type Baslikli<number> with value ["baslik", 1, 2]. Print the first element with y[0] using console.log.

Show example solution

type Baslikli<T> = [string, ...T[]];
const y: Baslikli<number> = ["baslik", 1, 2];
console.log(y[0]);

Test yourself

Which part of a variadic tuple takes the spread operator?
- Every element
- No spread operator used
- The first element
- The last element
Show answer

The last element — In a variadic tuple the spread operator is used on the last element; this keeps the leading elements fixed and the rest flexible in count.

Spread operator

type Baslikli<T> = [string, ____ T[]];

Show answer

type Baslikli<T> = [string, ... T[]];

Variadic tuple concepts

...

Spread operator
[A, ...B[]]

Variadic tuple form
son eleman

Spread here
[...A, ...B]

Tuple concatenation

Next lesson: Tuple inference

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