No. 278 · September 30, 2026 · Moderate

Camelize the keys

Implement CamelKeys<T> so string keys written in snake_case become camelCase. Values and non-string keys stay untouched.

Puzzle

Par 3

camelize-keys.ts

type CamelKey<S extends string> =
  S extends `${infer H}_${infer R}` ? `${H}${Capitalize<CamelKey<R>>}` : S

type CamelKeys<T> = ???

type CamelKeys<T> = ???

Stroke 1 of 3Not run yet

Replace ??? — your solution is checked against the cases below. Tab indents; press Escape then Tab to move focus out.

CamelKeys<{ first_name: string; user_id: number }>—
→ { firstName: string; userId: number }
CamelKeys<{ plain: true; two_words_here: 1 }>—
→ { plain: true; twoWordsHere: 1 }
CamelKeys<{ 0: 'zero'; snake_key: 'x' }>—
→ { 0: 'zero'; snakeKey: 'x' }

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

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

Fewest characters

No public scores yet.

This write-up publishes 24 hours after the puzzle closes.