store.js

// username → login count
const users = new Map();

export function authenticate(username) {
  let user = users.get(username);
  if (!user) {
    user = {
      name: username,
      loginCount: 0,
      browsers: new Set(),
    };
    users.set(username, user);
  }
  return user;
}

export function getUser(username) {
  return users.get(username);
}

export function recordLogin(username, browser) {
  const user = getUser(username);
  user.loginCount++;
  user.browsers.add(browser);
}

export function shouldConfirm(username, browser) {
  const user = getUser(username);

  // There needs to be at least 1 recognized browser
  // for there to be an unrecognized browser.
  if (user.browsers.size < 1) return false;

  return !user.browsers.has(browser);
}