Simple JavaScript callback.

callback_sample.js

const users = [
  {Name: "Peter", Age: 30, Id: 1},
  {Name: "Bob", Age: 20, Id: 2},
  {Name: "Jack", Age: 25, Id: 3}
];

class userManager {
  constructor() {
    this.getById = (id, callback) => {
      let result = users.find( (u) => u.Id == id );
      if (result) {
        callback(result);
      } else {
        callback(`There is no users with id ${id}`);
      }
    }
  }
}

let um = new userManager();
let test1 = um.getById(3, (response) => console.log(response.Name));
let test2 = um.getById(5, (response) => console.log(response));