Upgrading from v1.x to v2 is fairly simple. Below are the major changes that are released as a part of CometChat Pro v2:

​
Initialization

The CometChat.init() method now takes an additional parameter. This parameter is an object of the AppSettings class. This object can be created by using the AppSettingsBuilder class. The AppSettings class provides the CometChat SDK with the below 2 details:
  1. Region of your app: The region of your app is the code of the region your app is hosted in. Currently, this can only hold one of the below two values: a. eu - If your app is hosted in Europe b. us - If your app is hosted in the United States of America.
  2. Presence Subscription.
//v1.x

var appID = "APP_ID";
CometChat.init(appID).then(
  () => {
    console.log("Initialization completed successfully");
  }, error => {
    console.log("Initialization failed with error:", error);
  }
);

//v2.x

var appId = "APP_ID";
let cometChatSettings = new CometChat.AppSettingsBuilder()
                          .subscribePresenceForAllUsers()
                          .setRegion("us")
                          .build();
CometChat.init(appId, cometChatSettings).then(
  () => {
    console.log("Initialization completed successfully");
  }, error => {
    console.log("Initialization failed with error:", error);
  }
);

​
Text Message Constructor

Since the only possible valid value the messageType parameter could take in the constructor was CometChatConstants.MESSAGE_TYPE_TEXT(text), we have excluded the messageType parameter from the TextMessage constructor. 
//v1.x

var textMessage = new CometChat.TextMessage(receiverID, messageText, messageType, receiverType);

//v2.x

var textMessage = new CometChat.TextMessage(receiverID, messageText, receiverType);

​
Mark a Message as Read

In v1.x, for marking the messages as read, every message had to marked as read individually using the CometChat.markMessageAsRead() which took the entire message object as the input parameter. Starting v2.0, the method name has been changed to markAsRead() which takes the messageId, receiverId, and receiverType as input. This method, will mark all the messages before the provided messageId for the conversation with the specified receiverId as read. For more information, please check here. 
//v1.x
CometChat.markMessageAsRead(message);

//v2.x
CometChat.markAsRead(messageId, receiverId, receiverType);

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Receive Real Time Delivery & Read Receipts

In v1.x, Real-time events for messages of a conversation being read or delivered can be obtained in the onMessageRead() and the onMessageDelivered() methods of the MessageListener class since in v1.x single messages would be marked as read. Starting v2.x, multiple messages are being read at a time. So the method names are changed a bit. onMessageRead() is changed to onMessagesRead() and onMessageDelivered() is changed to onMessagesDelivered(). 
//v1.x

let listenerId = "UNIQUE_LISTENER_ID";

CometChat.addMessageListener(
  "listenerId",
  new CometChat.MessageListener({
    onMessageDelivered: messageReceipt => {
      console.log("MessageDeliverd", { messageReceipt });
    },
    onMessageRead: messageReceipt => {
      console.log("MessageRead", { messageReceipt });
    }
  })
);


//v2.x

let listenerId = "UNIQUE_LISTENER_ID";

CometChat.addMessageListener(
  "listenerId",
  new CometChat.MessageListener({
    onMessagesDelivered: messageReceipt => {
      console.log("MessageDeliverd", { messageReceipt });
    },
    onMessagesRead: messageReceipt => {
      console.log("MessageRead", { messageReceipt });
    }
  })
);

​
Retrieve List of Friends

In v1.x to fetch only friends, you had to enable the Show only friends setting in the CometChat Dashboard. Starting v2.x, we have added a method in the UsersRequestBuilder to fetch only friends. This method accepts boolean and fetches only friends if it is set to true else it fetches all the users. By default it is set to false. For more information, please check here. 
//v2.x

let usersRequest = new UsersRequest.UsersRequestBuilder()
  .setLimit(limit)
  .friendsOnly(true)
  .build();