TypeScript · Function

Tuple Types

Tuple types describe fixed-position arrays: each index can have a different type, and the tuple can also express optional or rest elements. Labels improve readability, while readonly prevents mutation.

How it works

A tuple type describes an array with a known number of elements and a specified type at each position. It can include optional elements, a final rest element for additional values, labels for readability, and a readonly modifier to prevent writes.

Syntax

type Pair = [string, number];

Key points

Tuples encode both the collection's arity and the types associated with its positions.
Add ? after an element type to allow that position to be omitted.
A rest element makes the tuple open-ended, allowing additional values described by an array or tuple type.
Use readonly before the tuple syntax when callers should not mutate its indexed elements.

Common mistakes

Readonly tuples reject assignments through their indexed properties.

Example: Labeled readonly tuple

type UserRecord = readonly [id: number, name: string];

function formatUser(user: UserRecord): string {
  return `${user[0]}: ${user[1]}`;
}

const user: UserRecord = [42, "Ada"];
console.log(formatUser(user));

Defines a labeled, readonly tuple and reads each value using its position.

When to use it

Fixed-shape results

Representing a function result with a fixed positional shape, such as a status value paired with a payload.

Fixed prefix with variable tail

Typing APIs where the first few arguments are fixed and later arguments share an array element type.

Readonly records

Passing immutable positional data between layers of a large codebase.

Interview questions

QWhat distinguishes a tuple from an ordinary array?

A tuple is an array type whose length and element types at particular positions are known. For example, [string, number] requires a string at index 0 and a number at index 1.

Follow-up: How do you model additional values after a fixed prefix? Use a rest element such as [string, ...number[]].

QHow are optional tuple elements declared?

Optional tuple elements use ? after the element type, such as [string, number?]. An omitted optional element is represented in the type system as potentially absent.

Follow-up: Where may optional elements appear? An optional element must be followed only by optional elements.

QWhat does a readonly tuple change?

Prefix the tuple with readonly, for example readonly [string, number]. The tuple remains useful for reading, but assignments to its properties are rejected.

Follow-up: When is readonly a good default? The documentation recommends it when tuples are created and generally left unmodified.

Array types
ReadonlyArray
Optional tuple elements
Rest elements in tuple types
Labeled tuple elements

Source: Documentation - Object Types. Checked 2026-10-15.