No. 235 · August 18, 2026 · Moderate

PickByType

Implement `PickByType<T, U>` so it keeps only the properties whose value type is assignable to `U`. The rejected keys must disappear entirely, not survive as `never`.

Puzzle

Par 3

Puzzle

pick-by-type.ts

interface Mixed {
  id: number
  name: string
  active: boolean
  score: number
}

type PickByType<T, U> = ???

type PickByType<T, U> = ???

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.

Checks

PickByType<Mixed, number>—
→ { id: number; score: number }
PickByType<Mixed, string>—
→ { name: string }
PickByType<Mixed, symbol>—
→ {}
PickByType<{ a: string }, string>—
→ { a: string }

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

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Annotated solution

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