array_expert_change_elements.js

const users = [
  {
    name: "Jane",
    balance: 100.00
  },
  {
    name: "John",
    balance: 55.25
  }
];

// define pure methods for our `map`
const double = (amount) => amount * 2;
const doubleUserBalance = (user) => ({
  ...user,
  balance: double(user.balance),
});

// transform all the users
const usersWithDoubledBalance = users.map(doubleUserBalance);