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Javascript – Is it possible to create dynamic array type with generic?

I’m currently trying to make array type for my project

what I want to do is, whenever I give params to type, I want to make type base on it.

example:

 type Dimension = Number;
 type Point<Dimension, T extends Number[] = []> = Dimension extends T['length'] ? T : Point<Dimension, [...T, Number]>

 type Boundary = ??? // I wanna declare array of length 2N(double of Point<N>)


 //usage
 const x = [1,2,3] as Point<3>
 const xBoundary = [1,2,3,4,5,6] as Boundary<Point<3>>

 //usage2
 const y = [1,2,3,4] as Point<4>
 const yBoundary = [1,2,3,4,5,6,7,8] as Boundary<Point<4>>

Is this possible??

When I declare Point type, I’ve used recurisve type definition, so I’ve tried to use double recursion? and tried to spread Point type for Boundary type

e.g)

type Boundary<Point, Dimension, T extends Number[]> = Dimension extends T['length'] ? T : Point<Dimension, [...Point, ...T, Number]>;

But didn’t worked, since typescript doesn’t recognize Point as spread element and compiler accepts all Number[] types.

Tags:

2

Answers


  1. 0

    Yes, you can. Here is an example.

     type Point<Length extends number, Current extends number[] = []> = Current['length'] extends Length
  ? Current
  : Point<Length, [...Current, number]>;

 type Boundary<T extends number[]> = [...T,...T] 

 //usage
 const x = [1,2,3] as Point<3>
 const xBoundary = [1,2,3,4,5,6] as Boundary<Point<3>>

 //usage2
 const y = [1,2,3,4] as Point<4>
 const yBoundary = [1,2,3,4,5,6,7,8] as Boundary<Point<4>>
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  2. 0

    I am assuming you want to have a tuple type with twice the length, this is possible like this.

    type Tuple<
  TItem,
  TLength extends number,
  TArray extends any[] = [],
> = TArray["length"] extends TLength
  ? TArray
  : Tuple<TItem, TLength, [...TArray, TItem]>;

type DoubleLength<
  TN extends number,
  _Arr extends unknown[] = [],
> = _Arr["length"] extends TN
  ? [..._Arr, ..._Arr]["length"]
  : DoubleLength<TN, [unknown, ..._Arr]>;

type Test = Tuple<string, DoubleLength<2>>
//   ^? [string, string, string, string]
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