const userMap = new WeakMap();

const user1 = { name: 'Alice' };
const user2 = { name: 'Bob' };

userMap.set(user1, { visits: 10 });
userMap.set(user2, { visits: 5 });

// Later in the code...
const entries = [];
for (const obj of [user1, null, user2]) {
  if (obj !== null && userMap.has(obj)) {
    entries.push(userMap.get(obj).visits);
  }
}

console.log(entries.reduce((sum, visits) => sum + visits, 0));