KeysUnion custom utility type. create a union of strings out of keys in an interface where the value type match the given generic

keys-unions.ts

// create union out of properties in an interface who'se value type match then given value types (can be given a union of types to match) #codesnippet 


// Stack Overflow explanation https://stackoverflow.com/questions/54520676/in-typescript-how-to-get-the-keys-of-an-object-type-whose-values-are-of-a-given/66144780

export type KeysUnion<T, V> = { [K in keyof T]-?: T[K] extends V ? K : never }[keyof T];

//  Playground

interface Cool {
    ice: string;
    cold: string;
}

interface Foobar {
    fizz: string;
    buzz: Cool;
}

interface FoobarWithNumbers {
    fizz: string;
    buzz: Cool;
    apples: number;
}

const obj: Foobar = {
    fizz: "test",
    buzz: {
        ice: "top",
        cold: "kek"
    }
}

const objWithApples: FoobarWithNumbers ={
    fizz: "test",
    buzz: {
        ice: "top",
        cold: "kek"
    },
    apples: 5
}

// should error
function someFunction0(someObj: Foobar, key: string) {
    return someObj[key];
}

function someFunction1(someObj: Foobar, key: KeysUnion<Foobar, string>) {
    return someObj[key];
}

// FIXME ideally this would error since there is no number value present in Foobar
function someFunction2(someObj: Foobar, key: KeysUnion<Foobar, number>) {
    return someObj[key];
}

function someFunction2p1(someObj: Foobar, key: KeysUnion<FoobarWithNumbers, number>) {
    return objWithApples[key];
}

function someFunction3(someObj: Foobar, key: KeysUnion<Foobar, any>) {
    return someObj[key];
}

const x = someFunction1(obj, "fizz")

// should error
const y = someFunction1(obj, "nope")

// should error
const z = someFunction2(obj, "fizz")

const apples1 = someFunction2p1(objWithApples, "apples")

// should error
const fizz = someFunction2p1(objWithApples, "fizz")

// should error
const a = someFunction3(obj, "nope")

const b = someFunction3(obj, "fizz")

const c = someFunction3(obj, "buzz")

const explicitKey0: KeysUnion<Foobar, string> = "fizz"

// should fail
const explicitKey1: KeysUnion<Foobar, number> = "fizz"

// should fail
const explicitKey2: KeysUnion<Foobar, string> = "nope"

const explicitKey2p1: KeysUnion<FoobarWithNumbers, number> = "apples"

// should fail
const explicitKey3: KeysUnion<Foobar, string> = "buzz"