Object has no method Javascript

I don't know why I am getting this problem. I have used prototype before in javascript and it works fine but for some reason it is not working here:

test.html:

<script type="text/javascript">
        $(document).ready(function(){
        UserOptions("test");
    });
</script>

UserOptions.js:

 function UserOptions(username){
    ...
    var userOptions = document.createElement("div");
    userOptions.className = "userOptions";
    **this.createBtns(userOptions);**
    userContainer.appendChild(userOptions);


    contentCenter.appendChild(userContainer);
    contentCenter.appendChild(br);

    BuddyList();
    }

    UserOptions.prototype = {
        createBtns:function(parent){
            var self = this;
            /* Add Buddy Button */
            var addBtnContainer = document.crea开发者_StackOverflow中文版teElement("div");
            addBtnContinaer.className = "addBtnContainer";
    ...}

I keep getting the error Object has no method 'createBtns'

You forgot the new keyword. Now it's trying to call your constructor as a regular function, and this will point to window or whatever.

new UserOptions("Fred");