TypeScript implementation of Infinite Dimension Array

recursiveArray.ts

type RecursiveArray<T> = (T | RecursiveArray<T>)[]

type UtilizedRecursiveArray<T> = RecursiveArray<T> & {
    get(...indices: number[]): T | RecursiveArray<T>
    set(value: T, ...indices: number[]): T
}

function utilize<T>(rarr: RecursiveArray<T>): UtilizedRecursiveArray<T> {
    const res = rarr as any
    
    res.get = (...indices: number[]) => {
        let el: T | RecursiveArray<T> = res
        for (const index of indices) {
            if ((el as Array<any>)?.length == null) {
                throw new Error(`There is no array in given index (${indices.join(', ')})`)
            }
            el = (el as RecursiveArray<T>)[index]
        }
        return el
    }
    res.set = (value: T, ...indices: number[]) => {
        let el: T | RecursiveArray<T> = res
        for (let i = 0; i < indices.length; ++i) {
            const index = indices[i]

            if (i === indices.length - 1) {
                (el as RecursiveArray<T>)[index] = value
                break
            }

            let child = (el as RecursiveArray<T>)[index]

            if (child === undefined) {
                child = (el as RecursiveArray<T>)[index] = []
            } else if ((child as Array<any>)?.length == null) {
                throw new Error(`There is already a non array element in index (${indices.join(', ')})`)
            }
            el = child as RecursiveArray<T>
        }
        return value
    }
    return res
}

Example

const deepArray =  utilize([1, [5, 3], [[4], 0]])

deepArray.get(2, 0, 0) // 4
deepArray.set(10, 1, 2) // now deepArray is [1, [5, 10], [[4], 0]])

Caveat

Don't use type undefined for type parameter T in UtilizedRecursiveArray<T>. There is no way to distinguish intended value or actual non existence of such array, in line 26. If you want to use, you have to give up for dynamic array creation during new indices path.

// Alternative implementation of set, to support `undefined` as their element
res.set = (value: T, ...indices: number[]) => {
  let el: T | RecursiveArray<T> = res
  for (let i = 0; i < indices.length; ++i) {
      const index = indices[i]

      if (i === indices.length - 1) {
          (el as RecursiveArray<T>)[index] = value
          break
      }

      let child = (el as RecursiveArray<T>)[index]

      if ((child as Array<any>)?.length == null) {
          throw new Error(`There is already a non array element in index (${indices.join(', ')})`)
      }
      el = child as RecursiveArray<T>
  }
  return value
}