jmpress.js zoom in and out with + or -

keyzoom.js

/**
 * Viewport Key Zoom In & Out
 * Press + and - to zoom the viewport in and out.
 */
(function() {
	'use strict';
	$.jmpress("defaults").viewPort.keyZoomAmount = 100;
	$.extend(true, $.jmpress('defaults').keyboard.keys, {
		187: 'zoomin' // plus
		,189: 'zoomout' // minus
	});
	$.jmpress("register", "zoomin", function() {
		var settings = $(this).jmpress('settings');
		if ( settings.viewPort.width ) {
			settings.viewPort.width += settings.viewPort.keyZoomAmount;
		}
		if ( settings.viewPort.height ) {
			settings.viewPort.height += settings.viewPort.keyZoomAmount;
		}
		$(this).jmpress("select", $(this).jmpress("active"), "resize");
	});
	$.jmpress("register", "zoomout", function() {
		var settings = $(this).jmpress('settings');
		if ( settings.viewPort.width ) {
			settings.viewPort.width -= settings.viewPort.keyZoomAmount;
		}
		if ( settings.viewPort.height ) {
			settings.viewPort.height -= settings.viewPort.keyZoomAmount;
		}
		$(this).jmpress("select", $(this).jmpress("active"), "resize");
	});
})();

check if the viewPort values are set. if not do not use 0. do not set them at all. leave them undefined.

if(settings.viewPort.width)
  settings.viewPort.width = ...;
if(settings.viewPort.height)
  settings.viewPort.height = ...;

settings.viewPort is ever set.
width and heigt may be false, which means window scale should not be considered in this dimension. It must stay false even when zooming

Thanks!