While browsing through some code, I stumbled upon this snippet -
type Record<K extends keyof any, T> = {
[P in K]: T;
};
I have a grasp on the fact that T> is a generic type assertion.
Could someone kindly elaborate on what K represents? Also, could you provide an explanation along with sample input/output for what this function returns?
Thank you in advance.
Initially, when you use keyof, it gives you a union of all keys within a type.
type BadVector = { x: string, y: boolean }
keyof BadVector // "x" | "y"
Hence, the statement K extends keyof any implies that K represents such a union type of keys. For instance, if you have the type keyof BadVector, it would extend keyof any.
Secondly, in a mapped type, every key in an object gets mapped to a new value type. Therefore, [P in K]: T signifies: Assign each property P in K to a value type T.
Consider the scenario provided in the documentation example:
interface CatInfo {
age: number;
breed: string;
}
type CatName = "miffy" | "boris" | "mordred";
const cats: Record<CatName, CatInfo> = {
miffy: { age: 10, breed: "Persian" },
boris: { age: 5, breed: "Maine Coon" },
mordred: { age: 16, breed: "British Shorthair" },
};
The datatype of cats is Record<CatName, CatInfo>. This indicates that for all types present in CatName, a corresponding value with the type CatInfo must be included.
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