---
title: Tuples
slug: /types/tuples
---

Tuple types represent a **fixed length** list, where the elements can have **different types**.
This is in contrast to [array types](../arrays), which have an unknown length and all elements have the same type.

## Tuple Basics

JavaScript array literal values can be used to create both tuple and array types:

```js flow-check
const arr: Array<number> = [1, 2, 3]; // As an array type
const tup: [number, number, number] = [1, 2, 3]; // As a tuple type
```

In Flow you can create tuple types using the `[type1, type2, type3]` syntax:

```js flow-check
const tuple1: [number] = [1];
const tuple2: [number, boolean] = [1, true];
const tuple3: [number, boolean, string] = [1, true, "three"];
```

When you get a value from a tuple at a specific index, it will return the
type at that index:

```js flow-check
const tuple: [number, boolean, string] = [1, true, "three"];

const num: number = tuple[0]; // Works!
const bool: boolean = tuple[1]; // Works!
const str: string  = tuple[2]; // Works!
```

Trying to access an index that does not exist results in an index-out-of-bounds error:

```js flow-check
const tuple: [number, boolean, string] = [1, true, "three"];

const none = tuple[3]; // Error!
```

If Flow doesn't know which index you are trying to access it will return all
possible types:

```js flow-check
const tuple: [number, boolean, string] = [1, true, "three"];

function getItem(n: number) {
  const val: number | boolean | string = tuple[n];
  // ...
}
```

When setting a new value inside a tuple, the new value must match the type at
that index:

```js flow-check
const tuple: [number, boolean, string] = [1, true, "three"];

tuple[0] = 2;     // Works!
tuple[1] = false; // Works!
tuple[2] = "foo"; // Works!

tuple[0] = "bar"; // Error!
tuple[1] = 42;    // Error!
tuple[2] = false; // Error!
```

## Strictly enforced tuple length (arity) {#toc-strictly-enforced-tuple-length-arity}

The length of the tuple is known as the "arity". The length of a tuple is
strictly enforced in Flow.

This means that a shorter tuple can't be used in place of a longer one:

```js flow-check
const tuple1: [number, boolean] = [1, true];

const tuple2: [number, boolean, void] = tuple1; // Error!
```

Also, a longer tuple can't be used in place of a shorter one:

```js flow-check
const tuple1: [number, boolean, void] = [1, true, undefined];

const tuple2: [number, boolean] = tuple1; // Error!
```

[Optional elements](#optional-tuple-elements) make the arity into a range.

## Tuples don't match array types {#toc-tuples-don-t-match-array-types}

Since Flow does not know the length of an array, an `Array<T>` type cannot be
passed into a tuple:

```js flow-check
const array: Array<number> = [1, 2];

const tuple: [number, number] = array; // Error!
```

Also a tuple type cannot be passed into to an `Array<T>` type, since then you
could mutate the tuple in an unsafe way (for example, `push`ing a third item onto it):

```js flow-check
const tuple: [number, number] = [1, 2];

const array: Array<number> = tuple; // Error!
```

However, you can pass it to a [`$ReadOnlyArray`](../arrays/#toc-readonlyarray) type, since mutation is disallowed:

```js flow-check
const tuple: [number, number] = [1, 2];

const array: $ReadOnlyArray<number> = tuple; // Works!
```

## Cannot use mutating array methods on tuples {#toc-cannot-use-mutating-array-methods-on-tuples}

You cannot use `Array.prototype` methods that mutate the tuple, only ones that do not:

```js flow-check
const tuple: [number, number] = [1, 2];
tuple.join(', '); // Works!

tuple.push(3); // Error!
```

## Length refinement

You can refine a [union](../unions) of tuples by their length:

```js flow-check
type Union = [number, string] | [boolean];
function f(x: Union) {
  if (x.length === 2) {
    // `x` is `[number, string]`
    const n: number = x[0]; // OK
    const s: string = x[1]; // OK
  } else {
    // `x` is `[boolean]`
    const b: boolean = x[0];
  }
}
```

## Tuple element labels

> NOTE: This and the following sections require your tooling to be updated as described in the "Adoption" section at the end of this page.

You can add a label to tuple elements. This label does not affect the type of the tuple element,
but is useful in self-documenting the purpose of the tuple elements, especially when multiple elements have the same type.

```js flow-check
type Range = [x: number, y: number];
```

The label is also necessary to add a variance annotation or optionality modifier to an element (as without the label we would have parsing ambiguities).

## Variance annotations and read-only tuples

You can add [variance](../../lang/variance)  annotations (to denote read-only/write-only) on labeled tuple elements, just like on object properties:

```js flow-check
type T = [+foo: number, -bar: string];
```

This allows you to mark elements as read-only or write-only. For example:

```js flow-check
function f(readOnlyTuple: [+foo: number, +bar: string]) {
  const n: number = readOnlyTuple[0]; // OK to read
  readOnlyTuple[1] = 1; // ERROR! Cannot write
}
```

You can also use the [`$ReadOnly`](../utilities/#toc-readonly)  on tuple types as a shorthand for marking each property as read-only:

```js flow-check
type T = $ReadOnly<[number, string]>; // Same as `[+a: number, +b: string]`
```

## Optional tuple elements

You can mark tuple elements as optional with `?` after an element’s label. This allows you to omit the optional elements.
Optional elements must be at the end of the tuple type, after all required elements.

```js flow-check
type T = [foo: number, bar?: string];
[1, "s"] as T; // OK: has all elements
[1] as T; // OK: skipping optional element
```

You cannot write `undefined` to the optional element - add `| void` to the element type if you want to do so:

```js flow-check
type T = [foo?: number, bar?: number | void];
declare const x: T;
x[0] = undefined; // ERROR
[undefined] as T; // ERROR

x[1] = undefined; // OK: we've added `| void` to the element type
```

You can also use the [`Partial`](../utilities/#toc-partial) and [`Required`](../utilities/#toc-required) utility types to make all elements optional or required respectively:

```js flow-check
type AllRequired = [number, string];
[] as Partial<AllRequired>; // OK: like `[a?: number, b?: string]` now

type AllOptional = [a?: number, b?: string];
[] as Required<AllOptional>; // ERROR: like `[a: number, b: string]` now
```

Tuples with optional elements have an arity (length) that is a range rather than a single number. For example, `[number, b?: string]` has an length of 1-2.

## Tuple spread

You can spread a tuple type into another tuple type to make a longer tuple type:

```js flow-check
type A = [number, string];
type T = [...A, boolean]; // Same as `[number, string, boolean]`
[1, "s", true] as T; // OK
```

Tuple spreads preserve labels, variance, and optionality. You cannot spread arrays into tuples, only other tuples.

At the value level, if you spread a tuple with optional elements into an array literal, then you cannot have anything after that spread and retain the tuple view of the array value.
That is because a tuple with optional elements has a length that's a range, so we don't know at what index any subsequent values would be at.
You can still type this value as the appropriate `Array<T>` type - only the tuple view of the value is affected.

```js flow-check
const a: [foo?: 1] = [];
const b = [0, ...a, 2]; // At runtime this is `[0, 2]`
b as [0, 1 | void, 2]; // ERROR
b as Array<number | void>; // OK

const c: [0, foo?: 1] = [0];
const d: [bar?: 2] = [2];
const e = [...c, ...d]; // At runtime this is `[0, 2]`
e as [0, foo?: 1, bar?: 2]; // ERROR
e as Array<number | void>; // OK
```

## Adoption

To use labeled tuple elements (including optional elements and variance annotations on elements) and tuple spread elements,
you need to upgrade your infrastructure so that it supports the syntax:

- `flow` and `flow-parser`: 0.212.0
- `prettier`: 3
- `babel` with `babel-plugin-syntax-hermes-parser`. See [our Babel guide](../../tools/babel/) for setup instructions.
- `eslint` with `hermes-eslint`. See [our ESLint guide](../../tools/eslint/) for setup instructions.
