No. 212 · July 20, 2026 · Gentle

Rebuild Pick

Implement `MyPick<T, K>` so it keeps only the keys named in `K`. This is the built-in `Pick`, written by hand — one mapped type, no conditionals.

Try the puzzle yourself

Par 2

Puzzle

rebuild-pick.ts

interface Todo {
  title: string
  description: string
  done: boolean
}

type MyPick<T, K extends keyof T> = ???

type MyPick<T, K extends keyof T> = ???

Stroke 1 of 2Not run yet

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

Checks

MyPick<Todo, 'title'>—
→ { title: string }
MyPick<Todo, 'title' | 'done'>—
→ { title: string; done: boolean }
MyPick<Todo, never>—
→ {}

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 July 21, 2026

The solution

type MyPick<T, K extends keyof T> = { [P in K]: T[P] }

The common wrong answer

type MyPick<T, K extends keyof T> = { [P in keyof T]: T[P] }

Mapping over `keyof T` copies the whole object — `K` is never consulted. The clause after `in` is the key set you are building, not the one you are reading from.

Line by line

{ [P in K]: ... }
`K` is a union of literal keys. A mapped type distributes over that union, producing one property per member.
T[P]
An indexed access. Because `K extends keyof T`, `P` is always a real key of `T`, so this never widens to `unknown`.

Takeaway

A mapped type reads `{ [Name in KeySet]: ValueType }`. Everything else in this track is a variation on that shape.

Uses

Mapped types Pick Omit keyof Partial Record Mapped types Make Omit catch typos Flatten an intersection Require at least one property Make two prop shapes mutually exclusive TS2344 TS2536