No. 234 — TupleToUnion — Annotated solution

TupleToUnion

Implement TupleToUnion<T> so it turns a tuple into the union of its element types. The whole thing is one indexed access.

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Par 2

Puzzle

tuple-to-union.ts

type TupleToUnion<T extends readonly unknown[]> = ???

type TupleToUnion<T extends readonly unknown[]> = ???

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Replace ??? — your solution is checked against the cases below. Tab indents; press Escape then Tab to move focus out.

Checks

TupleToUnion<[1, 2, 3]>—
→ 1 | 2 | 3
TupleToUnion<['a']>—
→ 'a'
TupleToUnion<[]>—
→ never

How a check is judged Exact type equality, not assignability — an intersection is not the same as the flattened object.

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Annotated solution

Published August 18, 2026

The solution

type TupleToUnion<T extends readonly unknown[]> = T[number]

The common wrong answer

type TupleToUnion<T extends readonly unknown[]> =
  T extends readonly [infer H, ...infer R] ? H : never

A reasonable instinct — destructure and take the head — but it stops there and returns only the first element. Recursing and unioning the results would work, and it is exactly the long way round: T[number] already does it.

Line by line

T[number]
An indexed access whose key is the whole number type. Since every valid numeric index selects one element, asking for all of them at once yields the union of every element type.
TupleToUnion<[]>
The empty tuple has no valid numeric index, so the union of zero things comes back — and the union of nothing is never.

Takeaway

Reach for recursion only when indexing cannot express the question. T[number], keyof T and T[keyof T] answer a surprising share of tuple and object problems in a single line.