How to declare a Fixed length Array in TypeScript
At the risk of demonstrating my lack of knowledge surrounding TypeScript types - I have the following question.
When you make a type declaration for an array like this...
position: Array<number>;
...it will let you make an array with arbitrary length. However, if you want an array containing numbers with a specific length i.e. 3 for x,y,z components can you make a type with for a fixed length array, something like this?
position: Array<3>
Any help or clarification appreciated!
The javascript array has a constructor that accepts the length of the array:
let arr = new Array<number>(3);
console.log(arr); // [undefined × 3]
However, this is just the initial size, there's no restriction on changing that:
arr.push(5);
console.log(arr); // [undefined × 3, 5]
Typescript has tuple types which let you define an array with a specific length and types:
let arr: [number, number, number];
arr = [1, 2, 3]; // ok
arr = [1, 2]; // Type '[number, number]' is not assignable to type '[number, number, number]'
arr = [1, 2, "3"]; // Type '[number, number, string]' is not assignable to type '[number, number, number]'
The Tuple approach :
This solution provides a strict FixedLengthArray (ak.a. SealedArray) type signature based in Tuples.
Syntax example :
// Array containing 3 strings
let foo : FixedLengthArray<[string, string, string]>
This is the safest approach, considering it prevents accessing indexes out of the boundaries.
Implementation :
type ArrayLengthMutationKeys = 'splice' | 'push' | 'pop' | 'shift' | 'unshift' | number
type ArrayItems<T extends Array<any>> = T extends Array<infer TItems> ? TItems : never
type FixedLengthArray<T extends any[]> =
Pick<T, Exclude<keyof T, ArrayLengthMutationKeys>>
& { [Symbol.iterator]: () => IterableIterator< ArrayItems<T> > }
Tests :
var myFixedLengthArray: FixedLengthArray< [string, string, string]>
// Array declaration tests
myFixedLengthArray = [ 'a', 'b', 'c' ] // ✅ OK
myFixedLengthArray = [ 'a', 'b', 123 ] // ✅ TYPE ERROR
myFixedLengthArray = [ 'a' ] // ✅ LENGTH ERROR
myFixedLengthArray = [ 'a', 'b' ] // ✅ LENGTH ERROR
// Index assignment tests
myFixedLengthArray[1] = 'foo' // ✅ OK
myFixedLengthArray[1000] = 'foo' // ✅ INVALID INDEX ERROR
// Methods that mutate array length
myFixedLengthArray.push('foo') // ✅ MISSING METHOD ERROR
myFixedLengthArray.pop() // ✅ MISSING METHOD ERROR
// Direct length manipulation
myFixedLengthArray.length = 123 // ✅ READ-ONLY ERROR
// Destructuring
var [ a ] = myFixedLengthArray // ✅ OK
var [ a, b ] = myFixedLengthArray // ✅ OK
var [ a, b, c ] = myFixedLengthArray // ✅ OK
var [ a, b, c, d ] = myFixedLengthArray // ✅ INVALID INDEX ERROR
(*) This solution requires the noImplicitAny typescript configuration directive to be enabled in order to work (commonly recommended practice)
The Array(ish) approach :
This solution behaves as an augmentation of the Array type, accepting an additional second parameter(Array length). Is not as strict and safe as the Tuple based solution.
Syntax example :
let foo: FixedLengthArray<string, 3>
Keep in mind that this approach will not prevent you from accessing an index out of the declared boundaries and set a value on it.
Implementation :
type ArrayLengthMutationKeys = 'splice' | 'push' | 'pop' | 'shift' | 'unshift'
type FixedLengthArray<T, L extends number, TObj = [T, ...Array<T>]> =
Pick<TObj, Exclude<keyof TObj, ArrayLengthMutationKeys>>
& {
readonly length: L
[ I : number ] : T
[Symbol.iterator]: () => IterableIterator<T>
}
Tests :
var myFixedLengthArray: FixedLengthArray<string,3>
// Array declaration tests
myFixedLengthArray = [ 'a', 'b', 'c' ] // ✅ OK
myFixedLengthArray = [ 'a', 'b', 123 ] // ✅ TYPE ERROR
myFixedLengthArray = [ 'a' ] // ✅ LENGTH ERROR
myFixedLengthArray = [ 'a', 'b' ] // ✅ LENGTH ERROR
// Index assignment tests
myFixedLengthArray[1] = 'foo' // ✅ OK
myFixedLengthArray[1000] = 'foo' // ❌ SHOULD FAIL
// Methods that mutate array length
myFixedLengthArray.push('foo') // ✅ MISSING METHOD ERROR
myFixedLengthArray.pop() // ✅ MISSING METHOD ERROR
// Direct length manipulation
myFixedLengthArray.length = 123 // ✅ READ-ONLY ERROR
// Destructuring
var [ a ] = myFixedLengthArray // ✅ OK
var [ a, b ] = myFixedLengthArray // ✅ OK
var [ a, b, c ] = myFixedLengthArray // ✅ OK
var [ a, b, c, d ] = myFixedLengthArray // ❌ SHOULD FAIL
Actually, You can achieve this with current typescript:
type Grow<T, A extends Array<T>> = ((x: T, ...xs: A) => void) extends ((...a: infer X) => void) ? X : never;
type GrowToSize<T, A extends Array<T>, N extends number> = { 0: A, 1: GrowToSize<T, Grow<T, A>, N> }[A['length'] extends N ? 0 : 1];
export type FixedArray<T, N extends number> = GrowToSize<T, [], N>;
Examples:
// OK
const fixedArr3: FixedArray<string, 3> = ['a', 'b', 'c'];
// Error:
// Type '[string, string, string]' is not assignable to type '[string, string]'.
// Types of property 'length' are incompatible.
// Type '3' is not assignable to type '2'.ts(2322)
const fixedArr2: FixedArray<string, 2> = ['a', 'b', 'c'];
// Error:
// Property '3' is missing in type '[string, string, string]' but required in type
// '[string, string, string, string]'.ts(2741)
const fixedArr4: FixedArray<string, 4> = ['a', 'b', 'c'];
EDIT (after a long time)
This should handle bigger sizes (as basically it grows array exponentially until we get to closest power of two):
type Shift<A extends Array<any>> = ((...args: A) => void) extends ((...args: [A[0], ...infer R]) => void) ? R : never;
type GrowExpRev<A extends Array<any>, N extends number, P extends Array<Array<any>>> = A['length'] extends N ? A : {
0: GrowExpRev<[...A, ...P[0]], N, P>,
1: GrowExpRev<A, N, Shift<P>>
}[[...A, ...P[0]][N] extends undefined ? 0 : 1];
type GrowExp<A extends Array<any>, N extends number, P extends Array<Array<any>>> = A['length'] extends N ? A : {
0: GrowExp<[...A, ...A], N, [A, ...P]>,
1: GrowExpRev<A, N, P>
}[[...A, ...A][N] extends undefined ? 0 : 1];
export type FixedSizeArray<T, N extends number> = N extends 0 ? [] : N extends 1 ? [T] : GrowExp<[T, T], N, [[T]]>;