const users = [
  { id: 1, name: 'Sarah' },
  { id: 2, name: 'Miguel' },
  { id: 3, name: 'Jordan' }
];

const activeUsers = new WeakSet();
activeUsers.add(users[0]);
activeUsers.add(users[2]);

users.splice(1, 1); // Remove Miguel

let count = 0;
for (const user of users) {
  if (activeUsers.has(user)) count++;
}

console.log(count);