index.ts

const map = <T, R = T>(items: T[], cb: (item: T) => R): R[] => {
  const mapped: R[] = [];

  for (let item of items) {
    mapped.push(cb(item));
  }

  return mapped;
};

const mapx = (items, cb) => {
  const mapped = [];

  for (let item of items) {
    mapped.push(cb(item));
  }

  return mapped;
};

console.log(map(['1', '2', '3'], str => parseInt(str))[0]);

interface Person {
  age: number;
  name: string;
}

interface Human {
  years: number;
}

map<Human, Person>([{ years: 12 }], ({ years: age }) => ({ age, name: 'unknown' }));
map([{ years: 12 }], ({ years: age }) => ({ age }));