array_expert_find_index.js

const users = [
  {
    id: 1,
    username: "supercoder",
  },
  {
    id: 2,
    username: "xXxSniperElitexXx",
  },
  {
    id: 3,
    username: "sPoNgEbOb_cAsE",
  }
];

const withId = (id) => (user) => user.id === id;
const userWithId = users.findIndex(withId(1)); // 0