Getting the Correct Nested Type in TypeScript Conditional Types for Iterables

In my quest to create a type called GoodNestedIterableType, I aim to transform something from 

Iterable<Iterable<A>>
to just A.

To illustrate, let's consider the following code snippet:

const arr = [
  [1, 2, 3],
  [4, 5, 6],
]

type GoodNestedIterableType<A> = A extends Iterable<infer B> 
  ? B extends Iterable<infer C>
    ? C : never
  : never
type GoodExtractedType = GoodNestedIterableType<typeof arr> // number

type BadNestedIterableType<A> = A extends Iterable<Iterable<infer B>>
  ? B
  : never
type BadExtractedType = BadNestedIterableType<typeof arr> // unknown

The GoodExtractedType correctly resolves to number, but surprisingly, the BadExtractedType ends up as unknown.

Despite expectations, even this scenario functions as intended:

BadNestedIterableType<Iterable<Iterable<number>> // number

I am seeking insights into the underlying mechanics of this phenomenon. Can anyone shed light on this behavior?

javascripttypescripttypesconditional-types

Answer №1

If you're looking to work with arrays, you can simply utilize the built-in [].flat or FlatArray type

interface Array<T> {
    /**
     * Generates a new array by concatenating all sub-array elements recursively up to the specified depth.
     *
     * @param depth The maximum recursion depth
     */
    flat<A, D extends number = 1>(
        this: A,
        depth?: D
    ): FlatArray<A, D>[]
}

type FlatArray<Arr, Depth extends number> = {
    "done": Arr,
    "recur": Arr extends ReadonlyArray<infer Inner Arr>
        ? FlatArray<InnerArr, [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20][Depth]>
        : Arr
}[Depth extends -1? "done": "recur"];

Type inference can be challenging, so I assume this feature is not implemented for simplicity and because there hasn't been a demand for it.

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