Uncaught Type error cannot assign to read only property

Hi,
I am getting the following error in console :

This is the code that causes it :

 var event = new Event('select');
        event.chat = chat;
        if (name) {
            event.chatName = name;
        }
        self.dispatchEvent(event);

Select allOpen in new window

previously i was using the following code but then i found that the initEvent thing is deprecrated :

var event = document.createEvent('Event');
        event.initEvent('select', true, true);
        event.chat = chat;
        if (name) {
            event.chatName = name;
        }
        self.dispatchEvent(event);

Select allOpen in new window

dispatchEvent = function (event) {
    if (!(event.type in this.listeners)) {
        return;
    }
    var stack = this.listeners[event.type];
    event.target = this;
    for (var i = 0, l = stack.length; i < l; i++) {
        stack[i].call(this, event);
    }

Select allOpen in new window

This is the code that gets called when an event is dispatched...
And the line : event.target = this;  is the place where error is thrown....

Why this read only error is getting thrown... However if i put a break point at any line or otherwise also write in console : event.target = this; or anything else... it doesnt throw any error .

Why is this read only error getting thrown.. All basically i am doing here is exposing a custom Event object... and have the ability to dispatch it...

Probably also i can do it in different way then constructing an Event object..

Thanks