Write a typesafe 'pick' function in typescript

Sounds like you might be looking for the Pick type. Would something like this work for you?

function pick<T, K extends keyof T>(obj: T, ...keys: K[]): Pick<T, K> {
  const ret: any = {};
  keys.forEach(key => {
    ret[key] = obj[key];
  })
  return ret;
}

const o = {a: 1, b: '2', c: 3}
const picked = pick(o, 'b', 'c');

picked.a; // not allowed
picked.b  // string
picked.c  // number

With Object.fromEntries (es2019):

function pick<T extends object, K extends keyof T> (base: T, ...keys: K[]): Pick<T, K> {
  const entries = keys.map(key => ([key, base[key]]));
  return Object.fromEntries(entries);
}