address-book.js

class AddressBook {
  constructor() {
    this.names = [];
    this.emails = [];
  }

  store(name, email) {
    this.names.push(name);
    this.emails.push(email);
  }

  lookup(name) {
    const index = this.names.indexOf(name);

    if (index >= 0) {
      return this.emails[index];
    } else {
      return 'address not found';
    }
  }
}

const myAddressBook = new AddressBook();
myAddressBook.store("bart", "[email protected]");
myAddressBook.store("maggie", "[email protected]");
console.log(myAddressBook.lookup("bart"));
// '[email protected]'
console.log(myAddressBook.lookup("homer"));
// 'address not found'

running-total.js

class RunningTotal {
  constructor() {
    this.currentTotal = 0;
  }
  add(n) {
    this.currentTotal += n;
  }
  getTotal() {
    return this.currentTotal;
  }
}
const runningTotal = new RunningTotal();
runningTotal.add(3);
runningTotal.add(7);
runningTotal.add(7);
console.log(runningTotal.getTotal());
// 17