Add degree property to GraphVertex.

src/data-structures/graph/GraphVertex.js

@@ -31,8 +31,9 @@ export default class GraphVertex {
   getNeighbors() {
     const edges = this.edges.toArray();
 
-    const neighborsConverter = ({ value }) => {
+    /** @param {LinkedListNode} node */
-      return value.startVertex === this ? value.endVertex : value.startVertex;
+    const neighborsConverter = (node) => {
+      return node.value.startVertex === this ? node.value.endVertex : node.value.startVertex;
     };
 
     // Return either start or end vertex.
@@ -47,6 +48,13 @@ export default class GraphVertex {
     return this.edges.toArray().map(linkedListNode => linkedListNode.value);
   }
 
+  /**
+   * @return {number}
+   */
+  getDegree() {
+    return this.edges.toArray().length;
+  }
+
   /**
    * @param {GraphEdge} requiredEdge
    * @returns {boolean}

src/data-structures/graph/__test__/GraphVertex.test.js

@@ -100,4 +100,26 @@ describe('GraphVertex', () => {
     expect(vertexA.findEdge(vertexB)).toEqual(edgeAB);
     expect(vertexA.findEdge(vertexC)).toBeNull();
   });
+
+  it('should calculate vertex degree', () => {
+    const vertexA = new GraphVertex('A');
+    const vertexB = new GraphVertex('B');
+
+    expect(vertexA.getDegree()).toBe(0);
+
+    const edgeAB = new GraphEdge(vertexA, vertexB);
+    vertexA.addEdge(edgeAB);
+
+    expect(vertexA.getDegree()).toBe(1);
+
+    const edgeBA = new GraphEdge(vertexB, vertexA);
+    vertexA.addEdge(edgeBA);
+
+    expect(vertexA.getDegree()).toBe(2);
+
+    vertexA.addEdge(edgeAB);
+    expect(vertexA.getDegree()).toBe(3);
+
+    expect(vertexA.getEdges().length).toEqual(3);
+  });
 });