This is a Vue.js sample.
The GoJS Diagram is held in a "diagram" component, and only exposes a small fraction of the functionality of Diagram. That includes the "modelData" property, to initialize the Diagram to show a model, and the "diagram" property, to get access to the Diagram.
That component also supports two different ways of modifying the diagram: access to the Model via the "model" method so that code can call Model methods such as Model.setDataProperty, and an "updateDiagramFromData" method. The latter permits data changes without calling Model methods to notify Diagrams showing the model, but at the cost of not being able to support undo and redo.
Note: this sample was developed with Vue.js version 1. For Vue 3 you may need to make some changes, such as using toRaw.