Types from both keys and values of object in Typescript

Question

I have two sets of string values that I want to map from one to the other as a constant object. I want to generate two types from that mapping: one for keys and one for values.

const KeyToVal = {
    MyKey1: 'myValue1',
    MyKey2: 'myValue2',
};

The keys are easy enough:

type Keys = keyof typeof KeyToVal;

I'm having trouble getting a compile-time type for the values. I thought maybe one of these would work:

type Values = typeof KeyToVal[Keys];
type Values<K> = K extends Keys ? (typeof KeyToVal)[K] : never;
type Prefix<
    K extends Keys = Keys, 
    U extends { [name: string]: K } = { [name: string]: K }
> = {[V in keyof U]: V}[K];

All of these just made Values to be string. I also tried adapting the two answers to How to infer typed mapValues using lookups in typescript?, but either I got my adaptations wrong, or the answers didn't fit my scenario in the first place.

Answer 1

The compiler will widen string literal type to string, unless some specific conditions are met as explained in github issues and PR, or const assertion is used for literal value. Const assertions appeared in TypeScript 3.4:

const KeyToVal = {
    MyKey1: 'myValue1',
    MyKey2: 'myValue2',
} as const;

type Keys = keyof typeof KeyToVal;
type Values = typeof KeyToVal[Keys]; //  "myValue1" | "myValue2"

Prior to 3.4, there was a workaround to get the same effect. To make the compiler infer literal types, you had to pass your object through a function with appropriately crafted generic type parameters, this one seems to do the trick for this case:

function t<V extends string, T extends {[key in string]: V}>(o: T): T {return o}

The whole purpose of this function is to capture and preserve types to enable type inference, it's entirely useless otherwise, but with it you can have

const KeyToVal = t({
    MyKey1: 'myValue1',
    MyKey2: 'myValue2',
});

type Keys = keyof typeof KeyToVal;
type Values = typeof KeyToVal[Keys]; //  "myValue1" | "myValue2"