array_expert_change_at_index.js

const users = [
  {
    name: "Jane",
    balance: 100,
  },
  {
    name: "John",
    balance: 75,
  },
  {
    name: "Ellis",
    balance: 31.3,
  },
];

const double = (amount) => amount * 2;
const indexToUpdate = 1; // we will change user at index 1 only
const nextUsers = users.map((user, index) =>
  index === indexToUpdate ? { ...user, balance: double(user.balance) } : user
);