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webtype.org#218 last-element · par 3

No. 218 · July 26, 2026 · Moderate

Last

Implement `Last<T>` so it returns the type of the final element of a tuple. An empty tuple has no last element — return `never`.

01

Try the puzzle yourself

Par 3

Puzzle

last-element.ts
Stroke 1 of 3Not run yet

Replace ??? — your solution is checked against the cases below.

Checks

4
  • Last<[1, 2, 3]>—
    → 3
  • Last<['only']>—
    → 'only'
  • Last<[]>—
    → never
  • Last<[string, number]>—
    → number

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

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02

Annotated solution

Published July 27, 2026

The solution

type Last<T extends readonly unknown[]> =
  T extends readonly [...unknown[], infer L] ? L : never

The common wrong answer

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

Indexing a tuple with `number` gives the union of *every* element type, so `[1, 2, 3]` yields `1 | 2 | 3`. Position information is lost the moment you index that way.

Line by line

  1. [...unknown[], infer L]

    A tuple pattern may contain exactly one rest element, and it is allowed to sit at the front. Matched against a fixed-length tuple, the rest absorbs everything except the final slot, which binds to `L`.

  2. : never

    The empty tuple matches nothing — there is no final slot to bind — so the false branch answers with the type that has no values.

Takeaway

A rest element in a tuple pattern does not have to be last. Leading rests are how you reach the end of a tuple without counting your way there.