hasEdge(vertexIdA, vertexIdB) {
    return this.vertexIdsToEdgeId[vertexIdA + '_' + vertexIdB] !== undefined;
  }
Check whether or not two vertices are connected by an edge.

@param {Number} vertexIdA A vertex id.
@param {Number} vertexIdB A vertex id.
@returns {Boolean} A boolean indicating whether or not two vertices are connected by an edge.