knockout throws Message: TypeError: <xxx> is not a function. What does it mean?

Question

I have this code:

<ul id="chats" data-bind="foreach: chats">
   <li>
      <div class="chat_response" data-bind="visible: CommentList().length == 0">
        <form data-bind="submit: $root.addComment">
           <input class="comment_field" placeholder="Comment…" data-bind="value: NewCommentText" />
        </form>
      </div>
      <a class="read_more_messages" data-bind="visible: moreChats, click: showMoreChats">Read More Messages...</a>
   </li>
</ul>

function ChatListViewModel(chats) {
   // Data
   var self = this;

   self.chats = ko.observableArray(ko.utils.arrayMap(chats, function (chat) {
       return { CourseItemDescription: chat.CourseItemDescription,
           CommentList: ko.observableArray(chat.CommentList),
           CourseItemID: chat.CourseItemID,
           UserName: chat.UserName,
           ChatGroupNumber: chat.ChatGroupNumber,
           ChatCount: chat.ChatCount,
           NewCommentText: ko.observable("")
       };
   }));

   self.moreChats = ko.observable(true);

   self.showMoreChats = function () {
       var LastChatGroupNumber = self.chats()[self.chats().length - 1].ChatGroupNumber;

       $.ajax({
           url: $.CourseLogic.dataitem.GetMoreChatsUrl,
           data: "{chatGroupNumber: " + ko.toJSON(LastChatGroupNumber + 1) + "}",
           type: "post",
           contentType: "application/json",
           success: function (chats) {
               var chatList = self.chats();
               $.each(chats, function (index, chat) {
                   self.chats.push(chat);
               });
           }
       });
   }

}

ko.applyBindings(new ChatListViewModel(initialData));

But I get this error when the showMoreChats function is called:

Unable to parse bindings. Message: TypeError: CommentList is not a function; Bindings value: visible: CommentList().length == 0

What does it mean?

Answer 1

It's not that CommentList is undefined, it's just that it's not an observable (hence not a function). The reason being that in your ajax callback, you are just pushing the 'chat' objects received from your server "as is". You're not creating for example a new observableArray called CommentList, but you're just putting a bare array CommentList - hence the thrown error by KO.

You would need to make the same transformation as you did when constructing self.chats in the viewmodel constructor, e.g.:

$.each(chats, function(index, chat) {
    self.chats.push(
        {
            CourseItemDescription: chat.CourseItemDescription,
            CommentList: ko.observableArray(chat.CommentList),
            CourseItemID: chat.CourseItemID,
            UserName: chat.UserName,
            ChatGroupNumber: chat.ChatGroupNumber,
            ChatCount: chat.ChatCount,
            NewCommentText: ko.observable("")
        }
    );
});

By the way, you should also take a look at the ko.mapping plugin, it can do this transformation for you.

Edit: Also, for better performance you shouldn't push each individual item into the observable array, but add them in one bulk operation, something like:

self.chats( self.chats().concat( ko.utils.arrayMap(chats, function(chat) {
    return { /* ... same as before ... */ };
} ) );