Dynamically generate return type based on parameter in TypeScript

Question

I have a function that receives an array of strings and returns an object whose keys are the strings and every value is undefined :

function getInitialCharacteristics(names: string[]): ??? {
  return names.reduce((obj, name) => ({ ...obj, [name]: undefined }), {});
}

Example usage:

const result = getInitialCharacteristics(["hello", "world"]);
// result == { hello: undefined, world: undefined }

Now I wonder how I can define the proper return value for getInitialCharacteristics using TypeScript. I'd have to use generics or somehow dynamically generate that type. Is that possible?

Answer 1

If you are going to call this function with constants or string literals, typescript can help you get a stricter type for the return object

function getInitialCharacteristics<T extends string>(names: T[]): Record<T, undefined>
function getInitialCharacteristics(names: string[]): Record<string, undefined> {
  return names.reduce((obj, name) => ({ ...obj, [name]: undefined }), {});
}

const result = getInitialCharacteristics(["hello", "world"]);
result.hello //ok
result.world //ok
result.helloo //err

Playground link