Deep Pick

pick.js

const typeOf = _ => Object.prototype.toString.call(_);

// Result can be used with db.collection.find : PARTIALLY
const arrayToSchema = keys =>
  keys.reduce((schema, _) => {
    if (_.indexOf('.') > -1) {
      const [key, rest] = _.split('.');
      schema[key] = {
        ...schema[key],
        ...arrayToSchema([rest]),
      };
    } else {
      schema[_] = true;
    }
    return schema;
  }, {});

const pick = (obj, schema) =>
  Object.keys(schema).reduce((acc, key) => {
    switch (typeOf(obj[key])) {
      case '[object Object]':
        acc[key] = pick(obj[key], schema[key]);
        break;
      case '[object Array]':
        acc[key] = obj[key].map(_ => pick(_, schema[key]));
        break;
      default:
        acc[key] = obj[key];
    }

    return acc;
  }, {});

const obj1 = {
  key1: 1,
  key2: 2,
  key3: 3,
};

const obj2 = {
  key1: 1,
  key2: 2,
  key3: [
    { key5: 5, key6: 6, key7: 7 },
    { key5: 8, key6: 9, key7: 10 },
    { key5: 11, key6: 12, key7: 13 },
  ],
  key4: 4,
};

const obj3 = {
  key1: 1,
  key2: {
    key3: 3,
    key4: 4,
    key5: 5,
  },
};

console.log(pick(obj1, arrayToSchema(['key1', 'key3'])));
console.log(
  pick(obj2, arrayToSchema(['key1', 'key3.key5', 'key3.key6', 'key4'])),
);
console.log(pick(obj3, arrayToSchema(['key1', 'key2.key3', 'key2.key4'])));