Question

i have just started to making class in javascript, this is my first class that i have created something like:

function MessageBox(parent){
    this.id = "msg-box";   // ID for the Box
    this.createElements(this.id);
    this.parentElement = parent;
};
MessageBox.prototype = {
    createElements: function(id){
        // Create Main Container
        this.containerElement = document.createElement('div');
        this.containerElement.setAttribute('id', this.id);
        this.parentElement.appendChild(this.containerElement);
    }
};

but when i call this file in my index.html file it returns the error createElements() is not a function..

here is my index.html as well:

<script type="text/javascript">
        window.onload = function(){
            var box = MessageBox('body');
            //box.start();
        }
    </script>

Answer 1

As your function is a constructor function, you want to use

var box = new MessageBox('body');
//        ^^^

Else, no instance is created and your constructor is executed on the default context object, the global window object (which has no createElements property). Have a look at MDN’s introduction to the this keyword and the new operator.