Typescript: map object value type for each key

Question

In the example code bellow:

const obj = {
  a: () => 1,
  b: () => '2'
}

type typeMap = Record<keyof typeof obj, (arg: unknown) => void>

const map: typeMap = {
  a: (numberArg) => numberArg + 1,
  b: (stringArg) => stringArg.length
}

I want Typescript to be aware of the function arguments types in the map object to be the same as the return type of each key in the original obj object. I know there is the ReturnType<> utility, but I don't know how to use it to map each prop from obj .

In summary, I want numberArg to be of type number and stringArg to be of type string .

You can open this code in Typescript Playground .

Answer 1

Use a mapped object type :

// alias for convenience
type ObjType = typeof obj;
type TypeMap = {
  [K in keyof ObjType]: (arg: ReturnType<ObjType[K]>) => unknown
};

Playground link