What does `type StringKeys = { [K in keyof T]: K extends string ? K : never }[keyof T]` extract?

Question

TypeScript Professional — Hard

What does `type StringKeys = { [K in keyof T]: K extends string ? K : never }[keyof T]` extract?

Accepted Answer

The code `type StringKeys = { [K in keyof T]: K extends string ? K : never }[keyof T]` extracts all keys of T that are string types, excluding number and symbol keys. It creates a new type by filtering out non-string keys from the original type T.