Online presence system in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the main component that tracks user status.

HLD

class PresenceTracker {
  constructor() {
    this.users = new Map();
  }

  updateStatus(userId, status) {
    this.users.set(userId, [1]);
  }
}

Drag options to blanks, or click blank then click option'

Athis

Bstatus

CuserId

Dusers

Attempts:

3 left

💡 Hint

Common Mistakes

Using userId instead of status as the value.

Trying to assign this or users instead of status.

The updateStatus method should store the status for the given userId in the users map.

2fill in blank

medium

Complete the code to check if a user is currently online.

HLD

isUserOnline(userId) {
  return this.users.get(userId) === [1];
}

Drag options to blanks, or click blank then click option'

A'away'

B'offline'

C'online'

D'busy'

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 'offline' instead of 'online'.

Using a status that means away or busy.

The method should return true if the user's status is 'online'.

3fill in blank

hard

Fix the error in the method that removes a user from the presence map.

HLD

removeUser(userId) {
  this.users.[1](userId);
}

Drag options to blanks, or click blank then click option'

Adelete

Bpop

Cdiscard

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove or pop which are not Map methods.

Using discard which is from other languages.

The correct method to remove a key from a JavaScript Map is delete.

4fill in blank

hard

Fill both blanks to create a method that returns all users currently online.

HLD

getOnlineUsers() {
  return [...this.users.entries()].filter(([[1], [2]]) => [2] === 'online').map(([userId]) => userId);
}

Drag options to blanks, or click blank then click option'

AuserId

Bstatus

Cuser

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping userId and status names.

Using incorrect variable names that don't match the pair.

The entries() method returns [userId, status] pairs. We filter by status === 'online' and map to userId.

5fill in blank

hard

Fill all three blanks to implement a method that returns a count of users by status.

HLD

countByStatus() {
  const counts = {};
  for (const [1] of this.users.values()) {
    counts[[2]] = (counts[[3]] || 0) + 1;
  }
  return counts;
}

Drag options to blanks, or click blank then click option'

Astatus

Duser

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'user' instead of 'status' as the loop variable.

Using different variable names for the key and lookup.

We iterate over each status value, then increment the count for that status in the counts object.