Addinga new argument to an existent funct(arg1, arg(n)...) or making a whole new function?

Like the Title i want to make a method(in this case 'addItem') same as this one but accepting one more parameter

(function (){return{
    addItem : function (type, des, val) {
        var newItem, ID;

        ID = dataStructure.allItems[type][dataStructure.allItems[type].length - 1].id + 1;

        if (dataStructure.allItems[type].length > 0){
            ID = dataStructure.allItems[type][dataStructure.allItems[type].length - 1].id + 1
        }else {
            ID = 0;
        }


        //Create new ELEMENT based on 'exp' or 'inc' type
        if (type === 'exp'){
            newItem = new Expense(ID, des, val);
        }else if (type === 'inc'){
            newItem = new Income(ID, des, val);
        }

        //Push it into our data structure
        dataStructure.allItems[type].push(newItem)

        //Return the new element
        return newItem;
    }
};})();

I want to call( depending on some inputs) a method('addItem') from an object or invoke the other if the value of the input is 'X' .