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Update Pick to use new key remapping in mapped types

See original GitHub issue

Search Terms

pick omit key remapping


Update Pick in lib.d.ts to be:

type Pick<T, K extends keyof T> = {
    [P in keyof T as Extract<P, K>]: T[P];

It is currently defined as:

type Pick<T, K extends keyof T> = {
    [P in K]: T[P];

Use Cases

With this new definition, typescript can statically analyze the keys, which allows interfaces to extends these types.


interface Base {
    a: number,
    b: string,
    c: boolean,

// Errors: "An interface can only extend an object type or intersection of object types with statically known members"
interface PickBase0<K extends keyof Base> extends Pick<Base, K> {
    pickedKeys: K

type Pick1<T, K extends keyof T> = { [L in keyof T as Extract<L, K>]: T[L] }

// Works
interface PickBase1<K extends keyof Base> extends Pick1<Base, K> {
    pickedKeys: K

Playground Link


My suggestion meets these guidelines:

  • This wouldn’t be a breaking change in existing TypeScript/JavaScript code
  • This wouldn’t change the runtime behavior of existing JavaScript code
  • This could be implemented without emitting different JS based on the types of the expressions
  • This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
  • This feature would agree with the rest of TypeScript’s Design Goals.

Issue Analytics

  • State:open
  • Created 3 years ago
  • Reactions:1
  • Comments:5 (4 by maintainers)

github_iconTop GitHub Comments

tjjfvicommented, May 28, 2021

@bfricka The problem in that scenario boils down to Omit’s definition:

type Omit<T, K> = Pick<T, Exclude<keyof T, K>>

Because A has an index signature, that becomes Pick<A, string | number>. Homomorphic mapped types, on the other hand, have special casing for index signatures that maps them correctly.

#41966 looks like it might be relevant to your scenario.

bfrickacommented, Mar 24, 2021

For one thing I noticed this fixes issues with Omit or Pick + intersection types in some cases. I’m not smart enough to know what this is called, but when a Pick or Omit type is fed into certain generics, it gets reduced or simplified in ways I don’t fully understand. I’ve seen it in various cases, but here’s a playground example.

You can see that passing an omit type into a remapped type causes an issue, but this new way of doing it fixes the issue. I wish I understood why, but perhaps someone could shed some light on it.

Read more comments on GitHub >

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