Extract generic values from a known object

I'm attempting to build a function that accepts two objects, one object I know the type of, the other will be generic. Is it possible to extract the generic properties from the known object?

I've kind of hit a wall on this one, but here's what I've tried:

interface IPerson {
  name: string;
  age: number;
  height: number;
  weight: number;
  job: string;
}

interface IAgeAndJob {
  age: number;
  job: string;
}

interface IHeightAndWeight {
  height: number;
  weight: string;
}

const person: IPerson = {
  name: 'Leon Kennedy',
  age: 21,
  height: 5.11,
  weight: 165,
  job: 'Police Officer'
};

const mapProperties = <IPerson, T>(person: IPerson): T => {

  let obj = {
    // extract properties from person where they exist
  } as T;
  console.log(obj);

  return obj;
};

const result = mapProperties<IPerson, IAgeAndJob>(person);

If I was doing this with a known type I would just grab the keys using Object.keys and iterate over the person object and grab what I need. Is there a way of doing this?

You can't do it in exactly the way you've shown. To see why, let's look at what it would be without any types:

const mapProperties = (person) => {
  let obj = {
    // extract properties from person where they exist
  };
  console.log(obj);

  return obj;
};

You see the problem: There's no runtime information about what properties you want to extract, so you can't write runtime code to extract them.

The closest you can come is a function where you tell it what properties you want:

const mapProperties = <ObjectType extends IPerson, Keys extends keyof IPerson>(
    person: ObjectType,
    keys: Keys[]
): Pick<ObjectType, Keys> => {
    const obj: Partial<ObjectType> = {};
    for (const key of keys) {
        obj[key] = person[key];
    }
    return obj as Pick<ObjectType, Keys>;
};

const result = mapProperties(person, ["age", "job"]);
console.log(result);

Playground link

I kept the object type generic because you'd used a generic type parameter, but if this is only for IPerson, you can remove ObjectType and just use IPerson directly:

const mapProperties = <Keys extends keyof IPerson>(
    person: IPerson,
    keys: Keys[]
): Pick<IPerson, Keys> => {
    const obj: Partial<IPerson> = {};
    for (const key of keys) {
        obj[key] = person[key];
    }
    return obj as Pick<IPerson, Keys>;
};

Playground link

This begs the question: Where do we get the ["age", "job"] array from?

You can't get it from IAgeAndJob, but you can flip that on its head and get IAgeAndJob and the array from a common source — a model object:

const ageAndJobModel = {
    age: 42,
    job: "programmer",
};

type IAgeAndJob = typeof ageAndJobModel;
const ageAndJobKeys = Object.keys(ageAndJobModel) as (keyof IAgeAndJob)[];

// Using it elsewhere
const result = mapProperties(person, ageAndJobKeys);
console.log(result);

Playground link