Make function param accepts only keys from variable object

Question

I have an object with defined type:

type Type = { [key: string]: ... }

const variable: Type = {
    key1: ...,
    key2: ...,
    key3: ...,
}

I have a function, and I want that function only accept string with values from variable's keys:

func('key1')     // OK
func('key2')     // OK
func('key3')     // OK
func('keyother') // Error
func(3)          // Error

And this is what I have done:

type FuncAcceptParamFromAboveVariableKey = <K extends keyof typeof variable>(param: K) => any
const func: FuncAcceptParamFromAboveVariableKey = ...

But I can't achieve this yet:

• If I set as const variable: Type = {, K has string type and I can pass anything to func call
• If I set as const variable = {, I have achieve this but it make variable undefined type.

How can I achieve both of them without predefined Type with list of keys ([key1, key2, ...]). I don't want to maintain two list of the samething.

Typescript playground for this problem: link

Answer 1

You have to use helper function that does nothing just validates type.

const makeType = <T extends Type = Type>(obj: T) => obj

const variable = makeType({
    key1: 0,
    key2: 0,
    key3: 0,
})

Playground