code: function nodeHasNeighbor(graph, type, direction, node, neighbor) {
  
    const nodeData = graph._nodes.get(node);
  
    if (type !== 'undirected') {
  
      if (direction !== 'out' && typeof nodeData.in !== 'undefined') {
        for (const k in nodeData.in)
          if (k === neighbor)
            return true;
      }
      if (direction !== 'in' && typeof nodeData.out !== 'undefined') {
        for (const k in nodeData.out)
          if (k === neighbor)
            return true;
      }
    }
  
    if (type !== 'directed' && typeof nodeData.undirected !== 'undefined') {
      for (const k in nodeData.undirected)
          if (k === neighbor)
            return true;
    }
  
    return false;
  }
docs: Function returning whether the given node has target neighbor.
  
  @param  {Graph}        graph     - Target graph.
  @param  {string}       type      - Type of neighbor.
  @param  {string}       direction - Direction.
  @param  {any}          node      - Target node.
  @param  {any}          neighbor  - Target neighbor.
  @return {boolean}