No. 255 — First — Annotated solution

First

Implement First<T> so it produces the type of the first element of the tuple T, or never when T is empty.

Try the puzzle yourself

Par 2

Puzzle

head-tuple.ts

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

type First<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

First<[1, 2, 3]>—
→ 1
First<[]>—
→ never
First<[string]>—
→ string

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

How everyone did

Fewer than 5 people have solved this one so far. The distribution appears once there is enough of a sample to mean anything.

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Annotated solution

Published September 8, 2026

The solution

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

The common wrong answer

type First<T extends readonly unknown[]> = T extends [infer H] ? H : never

A one-element pattern matches one-element tuples and nothing else. First<[string]> works, First<[1, 2, 3]> falls straight through to never, and the puzzle looks half-solved because one check passes. The pattern has to say "and then anything".

Line by line

[infer H, ...unknown[]]
The rest is spread as unknown[] rather than inferred, because nothing here needs it. Naming what you will not use is the most common way these patterns get harder to read than they are.
: never
never is the honest answer for "there is no such element" — not undefined, which would claim there is an element and its value is missing.

Takeaway

Tuple patterns are exact. [H] means "a tuple of length one", and if you meant "at least one" you have to write the rest of it.