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.

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.