Zoom Box Plugin allows to zoom a part of projection.
Resource : sample source code
Learn : javascript zoom box plugin sources
Zoom Box
//create zoom box and register instance in projection.
var zoombox = new JenScript.ZoomBoxPlugin({
speed : 'fast', //slow, default, fast
zoomBoxDrawColor : 'cyan',
zoomBoxFillColor : 'pink'
});
proj.registerPlugin(zoombox);
//zoom box requires a lock
zoombox.select();