function resolveUrl( prefix, url ) {
    return prefix ? prefix + url : url;
}
<em>- Needs documentation -</em>

@event update-viewport
@memberof OpenSeadragon.Viewer
@type {object}
@property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised the event.
@property {?Object} userData - Arbitrary subscriber-defined object.