No. 222 · July 30, 2026 · Hard

Reverse

Implement `Reverse<T>` so it returns the tuple with its elements in the opposite order.

Try the puzzle yourself

Par 4

Puzzle

reverse-tuple.ts

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

type Reverse<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

Reverse<[1, 2, 3]>—
→ [3, 2, 1]
Reverse<[]>—
→ []
Reverse<['a']>—
→ ['a']
Reverse<[1, 'b', true]>—
→ [true, 'b', 1]

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 July 31, 2026

The solution

type Reverse<T extends readonly unknown[]> =
  T extends readonly [infer H, ...infer R] ? [...Reverse<R>, H] : []

The common wrong answer

type Reverse<T extends readonly unknown[]> =
  T extends readonly [infer H, ...infer R] ? [H, ...Reverse<R>] : []

This recurses correctly and rebuilds the tuple in the order it took it apart, so it is an elaborate identity function. Reversal is entirely a question of which side of the spread the head goes on.

Line by line

[infer H, ...infer R]
The classic head/tail split. `H` binds the first element, `R` binds a tuple of everything after it.
[...Reverse<R>, H]
The head is appended *after* the reversed tail. The first element of the input therefore ends up last, and every level of the recursion pushes its head one place further back.

Takeaway

Head-and-tail recursion rebuilds a tuple one element at a time; the order you reassemble in is the only thing that distinguishes a copy from a reversal, a filter or a map.

Uses

Recursion Get a value by a typed deep path Make an object deeply readonly TS2456 TS2589 TS7023