Get Middle Element

Problem Link

Problem

Get the middle element of the array by implementing a GetMiddleElement method, the result should be returned in an array format.

GetMiddleElement<[1, 2, 3, 4, 5]> // [3]
GetMiddleElement<[1, 2, 3, 4, 5, 6]> // [3, 4]  (two middle elements for even length)

Solution

Approach: Peel Both Ends Simultaneously

Remove one element from the front and one from the back each step. When 1 or 2 elements remain, that's the middle.

type GetMiddleElement<T extends unknown[]> =
  T extends [infer _First, ...infer Middle, infer _Last]
    ? Middle extends []
      ? T  // 2 elements left — both are middle
      : GetMiddleElement<Middle>
    : T   // 0 or 1 element — that's the middle

How it works:

1. Pattern match [First, ...Middle, Last] to peel first and last.
2. If Middle is empty, both First and Last remain — return T (the 2-element tuple).
3. Otherwise, recurse on Middle.
4. If T has 0 or 1 elements (can't destructure both ends), return T as-is.

Key Takeaways

• TypeScript's rest-in-the-middle pattern [infer F, ...infer M, infer L] is a powerful way to peel both ends at once.
• The invariant "middle is empty means exactly 2 elements left" works because we only reach this branch with ≥ 2 elements.