How to use the gojs.Part function in gojs

commitLayers: function () {
                // optional: add circles in the background
                // need to remove any old ones first
                let diagram = this.diagram;
                let gridlayer = diagram.findLayer('Grid');
                let circles = new go.Set(go.Part);
                gridlayer.parts.each((circle: any) => {
                    if (circle.name === 'CIRCLE') { circles.add(circle); }
                });
                circles.each(function (circle) {
                    diagram.remove(circle);
                });
                // add circles centered at the root
                // let $ = go.GraphObject.make;  // for conciseness in defining templates
                for (let lay = 1; lay <= this.maxLayers; lay++) {
                    let radius = lay * this.layerThickness;
                    let circle =
                        $(go.Part,
                            { name: 'CIRCLE', layerName: 'Grid' },
                            { locationSpot: go.Spot.Center, location: this.root.location },
                            $(go.Shape, 'Circle',
                                { width: radius * 2, height: radius * 2 },

{ stroke: '#333333',
                font: 'bold 14px sans-serif',
                margin: new go.Margin(0, 10, 0, 0) },
              new go.Binding('text', 'name')),
            $(go.TextBlock,
              { stroke: '#333333',
                font: 'bold 14px sans-serif' },
              new go.Binding('text', 'desc'))
          )

      // define the Node template, representing an entity
      this.diagram.nodeTemplate =
          $(go.Node, 'Auto', // the whole node panel
            { selectionAdorned: true,
              resizable: true,
              layoutConditions: go.Part.LayoutStandard & ~go.Part.LayoutNodeSized,
              fromSpot: go.Spot.AllSides,
              toSpot: go.Spot.AllSides,
              isShadowed: true,
              shadowColor: '#C5C1AA' },
            new go.Binding('location', 'location').makeTwoWay(),
            // whenever the PanelExpanderButton changes the visible property of the 'LIST' panel,
            // clear out any desiredSize set by the ResizingTool.
            new go.Binding('desiredSize', 'visible', () => new go.Size(NaN, NaN)).ofObject('LIST'),
            // define the node's outer shape, which will surround the Table
            $(go.Shape, 'Rectangle',
              { fill: this.getBrush('lightgrad'), stroke: '#756875', strokeWidth: 3 }),
            $(go.Panel, 'Table',
              { margin: 8, stretch: go.GraphObject.Fill },
              $(go.RowColumnDefinition, { row: 0, sizing: go.RowColumnDefinition.None }),
              // the table header
              $(go.TextBlock,

// need to remove any old ones first
                let diagram = this.diagram;
                let gridlayer = diagram.findLayer('Grid');
                let circles = new go.Set(go.Part);
                gridlayer.parts.each((circle: any) => {
                    if (circle.name === 'CIRCLE') { circles.add(circle); }
                });
                circles.each(function (circle) {
                    diagram.remove(circle);
                });
                // add circles centered at the root
                // let $ = go.GraphObject.make;  // for conciseness in defining templates
                for (let lay = 1; lay <= this.maxLayers; lay++) {
                    let radius = lay * this.layerThickness;
                    let circle =
                        $(go.Part,
                            { name: 'CIRCLE', layerName: 'Grid' },
                            { locationSpot: go.Spot.Center, location: this.root.location },
                            $(go.Shape, 'Circle',
                                { width: radius * 2, height: radius * 2 },
                                { fill: 'rgba(200,200,200,0.2)', stroke: null }));
                    diagram.add(circle);
                }
            }
        });