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.

Aplet123 · Accepted Answer · 2020-12-23 18:21:27Z

2

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

answered Dec 23, 2020 at 18:21

Aplet123

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4 Comments

Oswaldo Over a year ago

awesome! that worked, thank you very much for the fast answer

Aplet123 Over a year ago

@Oswaldo I'm glad I could help you! If you're satisfied, please mark the answer as accepted so people know that the question's been answered.

arlyon Over a year ago

Is there a solution that properly handles the type conversion for values? This solution loses information by setting the type for each mapped field to unknown.

40detectives Over a year ago

as @arlyon mentioned, original return types of each function are lost, is there a workaround to prevent that?

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