id.ts

/** https://docs.tsafe.dev/id  */
export const id = <T>(x: T) => x;

/**
 * Ensure that a that a specific type that we are declaring extends a more generic type
 *
 * Use case example 1:
 *
 * type MyObject = {
 *     p1: string;
 *     p2: string;
 *     a: string;
 *     b: string;
 * };
 *
 * We want to define a type that consist in an union of
 * all the property name that are letters:
 *
 * type AlphabeticalKeys = Id<keyof MyObject, "a" | "b">;
 *
 * Here AlphabeticalKeys is "a" | "b" but it's better than
 * simply writing it explicitly as we get autocompletion
 * and we can't include a property name that does not exist on MyObject.
 *
 * Use case example 2:
 *
 * We want to declare object type that only take string or number
 * as key value:
 *
 * export type MyObject = Id<Record<string, string | number>, {
 *     p1: string;
 *     p2: number;
 * }>;
 *
 * If later on someone adds "p3": string[] he will get a type error.
 * */
export type Id<Generic, Specific extends Generic> = Specific;