No. 264 — Everything but the first

Everything but the first

Implement Tail<T> so it produces the tuple T without its first element. An empty tuple stays empty.

Try the puzzle yourself

Par 3

Puzzle

drop-first.ts

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

type Tail<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

Tail<[1, 2, 3]>—
→ [2, 3]
Tail<[1]>—
→ []
Tail<[]>—
→ []

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 17, 2026

The solution

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

The common wrong answer

type Tail<T extends readonly unknown[]> = T extends [unknown, infer R] ? R : []

Without the spread, [unknown, infer R] describes a tuple of exactly two elements and R is the second *element*, not the rest of the tuple. Tail<[1, 2, 3]> matches nothing and falls to []; Tail<[1, 2]> would return 2 — a number where a tuple was expected.

Line by line

[unknown, ...infer R]
unknown in the head position matches any single element without naming it, and the spread makes R the remainder however long it is.
: []
The empty tuple has no head to remove, so it never matches the pattern and the fallback answers for it. Removing nothing from nothing is still nothing.

Takeaway

A spread in a tuple pattern is the difference between "the next one" and "all the others". Leaving it out silently changes what your pattern will even match.