JavaScript - Loss of scope in event handlers - self/this

Guys,

Its a common problem with a common soloution and I've used it loads myself... but I'm missing something probably quite obvious here in understanding why the solution actually works.

See following code, I'm creating and assigning an event handler and prior to creating the event handler I'm making a reference 'self' to the 'this' pointer so that I can access it when the event handler is called. Now I understand why the 'this' pointer is no longer in scope... what I don't understand is why a local variable, 'self', IS in scope when called from within the event handler. So if someone could clarify that small point for me I'llk be very happy...

Cheers Dale - Code follows...

    this.makeRequest = function()
    {
        if (this.Init())
        {
            this.request.Open(this.method, this.url, this.async);
            var self = this;
            this.request.onreadystatechange = function()
            {
                if(self.request.readyState == 4)
                {
                }
            }
            this.request.send(this.postData)
        }
        else alert ("Failed to open connection");
    }