Callback within a promise within a class?

I have a class that I use to load external resources via an XMLHttpRequest (this is for WebGL) so I'm loading models, shaders etc. My plan was to put a loading display up whilst it did all these requests and then when it's finally complete I want it to run a callback function from the original function that created it. However, I'm getting strange results when I try to run that call back (such as it has no access of any of the objects within the class that did the loading).

I can get around this problem by passing "this" into the loading class and then doing

self = this; 
promise(self.callback());

but I'd much rather specify the function that I want it to callback to after its done the loading. Does anyone know if this can be done? My code looks like this:

Main Class

this.loadingClass = new LoadingClass(this.LoadingComplete, resources);

Main.prototype.LoadingComplete = function()
{
    // Returns undefined if i specify the callback function instead of just "this"
    console.log(this.loadingClass.anyOfTheMembersOfThisClass);
}

Loading Class

LoadingClass = function(callback, resources) {

  ..

        Promise.all(resources).then(function(loadedResources)
        {
            ..
            callback();

        });
}