That depends on what object your user is resizing. Did you set Part.resizeObjectName to refer to a Shape? If you haven’t set that property, by default the user will be resizing the whole Node, which might not be what you want.
Take a look at the Shapes sample: http://gojs.net/latest/samples/shapes.html. First note that the node template sets resizable: true and resizeObjectName: “SHAPE”, where “SHAPE” is the name of the Shape object.
If you interactively resize the “Rectangle” or the “Ellipse” figure shapes, you can change the aspect ratio of the original shape. But if you hold down the Shift key, the ResizingTool will automatically keep width/height ratio the same.
However if you try resizing the “Square” or the “Circle” shapes, you’ll notice that you cannot make them into non-square or non-circular shapes. This is because the ResizingTool respects the Shape.geometryStretch property, if the ResizingTool.adornedObject (the Part.resizeObject) is a Shape.
But if what you are showing is not a Shape (maybe it’s a Picture or a Panel), then there is no Shape.geometryStretch property, so there’s no automatic respect for maintaining aspect ratio. However you could override ResizingTool.computeResize to always pass a false value for the reshape parameter when calling the base method.