Uncaught TypeError: object is not a function after second refresh

At the moment I'm facing a javascript problem. This is the javascript code:

$(document).ready(function() {
    /*
        createNewTodo("_OPDRACHT_", "_PRIO_", "_DONE_");

        _OPDRACHT_  =   wat er gedaan met worden.
        _PRIO_      =   prioriteit 1-5 (1 is hoogste, 5 is laagste)
        _DONE_      =   uitgevoerd 1-2 (1 is nee, 2 is ja
    */

    function todo(opdracht,prio,done) {
      this.opdracht = opdracht;
      this.prio = prio;
      this.done = done;
    }

    function createNewTodo(opdracht,prio,done) {
      var createdTodo = new todo(opdracht,prio,done);
      if ( localStorage.todoCount == undefined ) {
          localStorage.setItem('todoCount', 0) 
      }
      var todoSize = parseInt(localStorage.todoCount) + 1;
      commitToStorage(todoSize,createdTodo);
    }

    function addList(){
        var val = $('#name').val();
        var tjap = createNewTodo(val,"1","1");
    }

    function commitToStorage(objectCount,newObject) {

      var item = + objectCount;
      localStorage.setItem('todoCount', objectCount);

      localStorage.setItem(item, JSON.stringify(newObject));
    }

    var todoCount = localStorage.getItem('todoCount');  
    for (i=1;i<=todoCount;i++)
      {
        var number = parseInt(i) + 1;
        var todo = jQuery.parseJSON(localStorage.getItem(+ i));
        createMarkup(todo);
      } 

    function createMarkup(todo) {
      $('ul#items').append(
        "<li>" + 
          todo.opdracht + " " +
        "</li>"
      );
    }

    $('#add-btn').click(addList);

});

When I click on the add-btn it will insert a item in localstorage with a array of values. Everything is working fine until I refresh the page. At the first time I can add as many items in localstorage without a problem but when I try to add items after refreshing the page I get the following error: Uncaught TypeError: object is not a function. Been searching a lot on the internet but could't find a appropriate solution for this. Anyone that can help me out and push me in the right direction? Thanks in advance.