getEdges(vertexId) {
    let edgeIds = Array();
    let vertex = this.vertices[vertexId];

    for (var i = 0; i < vertex.neighbours.length; i++) {
      edgeIds.push(this.vertexIdsToEdgeId[vertexId + '_' + vertex.neighbours[i]]);
    }

    return edgeIds;
  }
Returns the ids of edges connected to a vertex.

@param {Number} vertexId A vertex id.
@returns {Number[]} An array containing the ids of edges connected to the vertex.