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.

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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> = ???

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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

Published August 19, 2026

The solution

type PickByType<T, U> = {
  [K in keyof T as T[K] extends U ? K : never]: T[K]
}

The common wrong answer

type PickByType<T, U> = {
  [K in keyof T]: T[K] extends U ? T[K] : never
}

The filtering happens on the value side, so every key survives — the rejected ones just hold never. PickByType<Mixed, number> becomes { id: number; name: never; active: never; score: number }, which is a very different type from the one asked for, and a nearly unusable one.

Line by line

as T[K] extends U ? K : never
The as clause rewrites each key as the mapping runs. Here the rewrite is conditional: keep the key unchanged when the value matches, or rename it to never when it does not.
never
A key of type never cannot exist, so TypeScript omits the property rather than creating an impossible one. That quiet rule is what turns key remapping into a filter.

Takeaway

Filtering an object type happens in the key clause, never in the value. as ... : never is the only way to make a property genuinely disappear.

Uses

Mapped types Pick Partial Omit Record Mapped types Make Omit catch typos Flatten an intersection Require at least one property Make two prop shapes mutually exclusive TS2536