Question

When I do a console trace on the positionSlides method its showing slideShow as undefined.
How can this be when I clearly instantiate it in the document.ready callback. I also make sure to make this variable global so both the slideShow and the slideShowNavigation would have access to both these variables.

var slideShow, slideShowNavigation;

    $(document).ready(function(){

        slideShow = new SlideShow( $('#header #slideshow'), 980 );
        slideShowNavigation = new SlideShowNavigation( $('#header').find("#leftArrow"), $('#header').find("#rightArrow") );

    });


    // SLIDE SHOW CLASS
    function SlideShow( divContainer, slideWidth ){
        // Check to make sure a new instance is created
        if( ! (this instanceof SlideShow) ) return new SlideShow();

        this.$imageContainer = divContainer;
        this.slideWidth = slideWidth;
        var maxImages =     this.$imageContainer.children().length;

        this.getMaxSlides = function(){
            return maxImages;
        }

        this.positionSlides();

    }

    SlideShow.prototype.positionSlides = function(){
        console.log('imageContainer = '+slideShow);
    }

    SlideShow.prototype.update = function( dir ){

    }


    // ARROW NAVIGATION FOR SLIDESHOW

    function SlideShowNavigation( left, right){
        if( ! (this instanceof SlideShowNavigation) ) return new SlideShowNavigation();

        //this.updateArrows( slideShow.$imageContainer.find(":first") );

    }

    SlideShowNavigation.prototype.updateArrows = function( item ){

    }

Answer 1

The problem is this:

slideShow = new SlideShow( $('#header #slideshow'), 980 );

That line calls your SlideShow function, which calls positionSlides(), which looks for slideShow, but slideShow isn’t defined until your SlideShow function returns.

Instead, in positionSlides() use this instead of slideShow to refer to the instance (this way your class is independent of the variable name.