keyof + generic + extends & type lookup

// keyof + generic + extends & type lookup

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

const person: IPerson = {
  name: "Hyun",
  age: 33,
};

const getProp = (obj: IPerson, key: string | number): string | number => {
  return obj[key]; // Error
};

const getProp1 = (obj: IPerson, key: keyof IPerson): IPerson[keyof IPerson] => {
  return obj[key];
};

const getProp2 = <T, K extends keyof T>(obj: T, key: K): T[K] => {
  return obj[key];
};

const setProp = (
  obj: IPerson,
  key: string | number,
  value: string | number
): void => {
  obj[key] = value; // Error
};

const setProp1 = (
  obj: IPerson,
  key: keyof IPerson,
  value: IPerson[keyof IPerson]
): void => {
  obj[key] = value; // Error
};

const setProp2 = <T, K extends keyof T>(obj: T, key: K, value: T[K]): void => {
  obj[key] = value;
};