TypeScript: type pattern not matched when using keyof instead of literal

Question

type a = { [K in keyof { "foo" }]: { foo: "bar" } extends { K } ? true : false }["foo"];
type b = { foo: "bar" } extends { "foo" } ? true : false;

a and b should both be type true according to my understanding, but a turns out to be false.
Could anyone help explain the reason? Or maybe it's a bug in TypeScript?
Note: tested under TypeScript 4.4.4

true_mogician · Accepted Answer · 2021-12-01 06:17:31Z

0

Well, I've reported this problem as an issue in TypeScript's official repository, and here's the reply from RyanCavanaugh, which solves this problem.

type a = { foo: "bar" } extends { "foo" } ? true : false;
type b = { [K in keyof { "foo" }]: { foo: "bar" } extends { K } ? true : false }["foo"];
The type being resolved here (annotated for additional clarity) is
{ foo: "bar" } extends { K: any } ? true : false
K in this position is a property name, not a reference to the key type. You could equally have written
type a = { foo: "bar" } extends { "foo" } ? true : false;
type b = { [K in keyof { "foo" }]: { foo: "bar" } extends { QQQ } ? true : false }["foo"];
Which is clearly correctly false

This is true as expected:
type b = { [K in keyof { "foo": any }]: ({ foo: "bar" } extends Record<K, unknown> ? true : false) }["foo"];

answered Dec 1, 2021 at 6:17

true_mogician

7571 gold badge9 silver badges25 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

TypeScript: type pattern not matched when using keyof instead of literal

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related