mapValues

Creates an object with the same keys as the provided object and values generated by running the provided function for each value.

Use Object.keys(obj) to iterate over the object's keys. Use Array.prototype.reduce() to create a new object with the same keys and mapped values using fn.

const mapValues = (obj, fn) =>
  Object.keys(obj).reduce((acc, k) => {
    acc[k] = fn(obj[k], k, obj);
    return acc;
  }, {});

const users = {
  fred: { user: "fred", age: 40 },
  pebbles: { user: "pebbles", age: 1 },
};
mapValues(users, (u) => u.age); // { fred: 40, pebbles: 1 }