Explain the difference between var and function.varName in Javascript

Question

I am working the this code and would like to know the difference between var Point = Geometry.Point and GelImageLaneViewer.viewers = []

Under what circumstances would I choose to define a var inside a function versus a function.varName?

Code snippet follows from working code :

var GelImageLaneViewer = (function() {
    "use strict";

    var Point = Geometry.Point,
        **Rect = Geometry.Rect**,
        harmony = false,
        isGel1D = false,
        states = {
            "identified" : {
                "label" : "Identified",
                "visible" : true
                },
            "notAnalyzed" : {
                "label" : "Not Analyzed"
                }
            };
    function GelImageLaneViewer(elem) {
    var spotToolbar,
        svgBox,
        spotToolbarBox,
        oldViewBox,
        newHeight;
    ImageViewer.prototype.setViewportPosition('107px', '5px');
    ImageViewer.prototype.setRegionsOfInterest($('.gel1d').attr('data-all-regionsofinterest'));
    ImageViewer.call(this, elem);
    if($('.gel1d').attr('class') === 'hsppImageViewer gel1d') {
        isGel1D = true;
    }
    this.states = {};
    svgBox = this.svg.getBoundingClientRect();
    this.svg.setAttribute(
        "height",
        svgBox.height + 'px'
    );
    **oldViewBox = Rect.fromViewBox(this.svg);**
    this.svg.setAttribute(
        "viewBox",
        [oldViewBox.x, oldViewBox.y, oldViewBox.width, svgBox.height].join(' ')
    );
    this.image.setAttribute('height', svgBox.height);
}

    GelImageLaneViewer.prototype = Object.create(ImageViewer.prototype);
    GelImageLaneViewer.prototype.constructor = ImageViewer.constructor;

    **GelImageLaneViewer.viewers = [];**
    GelImageLaneViewer.noConflict = function () {
        harmony = true;
    }
    window.addEventListener("DOMContentLoaded", function () {
        var images = document.querySelectorAll('.hsppImageViewer'),
            stop = images.length,
            i,
            viewer,
            src;
        if (!harmony) {
            for (i = 0; i < stop; i += 1) {
                viewer = new GelImageLaneViewer(images[i]);
                src = images[i].getAttribute('data-src');
                if (!!src) {
                    viewer.loadURI(src);
                }
                viewer.initDraw();
                **GelImageLaneViewer.viewers.push(viewer);**
            }
        }
    }, false);

    return GelImageLaneViewer;
}());

Answer 1

GelImageLaneViewer.viewers will add a property which can be accessed by anything with access to GelImageLaneViewer by asking for GelImageLaneViewer.viewers . Var viewers would create a property which can only be accessed inside the scope of GelImageLaneViewer itself, and wouldn't be visible to anything outside the scope.