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!
This question is related to
typescript
types
fixed-length-array
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]]>;
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)
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
Source: Stackoverflow.com