question

What do you mean by append to user data? Do you mean creating the new KVP (Key/Value pair) for in the user data OR append the content to a Key’s value?

If it is the former, you can use the UpdateUserData API directly, this API will create new KVPs and overwrites the old KVPs.

If it is the latter, you can do that in the following steps:

1. Get the Value of the key that you want to append new data to using GetUserData API.
2. Use the string.concat to add the new value to old value.
3. Use the UpdateUserData API to overwrite the old KVP.

The concat method should be used for string. The error “myData.concat is not a function” indicates that “myData” is not a string. In addition, the “oldRecord” in your code is an object rather than a string and cannot be used in the concat method. For example, if there is a key named “CustomKey” and you want to append a string “NewData” as the prefix of the value, a correct version could be:

handlers.UpdateUserData2 = function(args, context){
    // var myData = args.myData;
    var FriendPlayFabId = args.FriendPlayFabId;
    var result = server.GetUserData({
            PlayFabId:FriendPlayFabId
        });
    if(result.Data != null)
        {                      
            var oldRecord = result.Data.CustomKey.Value;
            var myDataCombinedJSONObj = "NewData".concat(oldRecord);
            var myData = {CustomKey: myDataCombinedJSONObj};
            
        }
    server.UpdateUserData({
        PlayFabId: FriendPlayFabId,
        Data: myData
    });    
    return {messageValue: "success"};
}