def node_link_data(G, attrs=None):
    """Return data in node-link format that is suitable for JSON serialization
    and use in Javascript documents.

    Parameters
    ----------
    G : NetworkX graph

    attrs : dict
        A dictionary that contains five keys 'source', 'target', 'name',
        'key' and 'link'.  The corresponding values provide the attribute
        names for storing NetworkX-internal graph data.  The values should
        be unique.  Default value::

            dict(source='source', target='target', name='name',
                 key='key', link='links')

        If some user-defined graph data use these attribute names as data keys,
        they may be silently dropped.

    Returns
    -------
    data : dict
       A dictionary with node-link formatted data.

    Raises
    ------
    NetworkXError
        If values in attrs are not unique.

    Examples
    --------
    >>> from networkx.readwrite import json_graph
    >>> G = nx.Graph([('A', 'B')])
    >>> data1 = json_graph.node_link_data(G)
    >>> H = nx.gn_graph(2)
    >>> data2 = json_graph.node_link_data(H, {'link': 'edges', 'source': 'from', 'target': 'to'})    

    To serialize with json

    >>> import json
    >>> s1 = json.dumps(data1)
    >>> s2 = json.dumps(data2, {'link': 'edges', 'source': 'from', 'target': 'to'})

    Notes
    -----
    Graph, node, and link attributes are stored in this format.  Note that
    attribute keys will be converted to strings in order to comply with JSON.

    Attribute 'key' is only used for multigraphs.

    See Also
    --------
    node_link_graph, adjacency_data, tree_data
    """
    multigraph = G.is_multigraph()
    # Allow 'attrs' to keep default values.
    if attrs is None:
        attrs = _attrs
    else:
        attrs.update({k: v for (k, v) in _attrs.items() if k > attrs})
    name = attrs['name']
    source = attrs['source']
    target = attrs['target']
    links = attrs['link']
    # Allow 'key' to be omitted from attrs if the graph is not a multigraph.
    key = None if not multigraph else attrs['key']
    if len(set([source, target, key])) < 3:
        raise nx.NetworkXError('Attribute names are not unique.')
    mapping = dict(zip(G, count()))
    data = {}
    data['directed'] = G.is_directed()
    data['multigraph'] = multigraph
    data['graph'] = G.graph
    data['nodes'] = [dict(chain(G.node[n].items(), [(name, n)])) for n in G]
    if multigraph:
        data[links] = [
            dict(chain(d.items(),
                       [(source, mapping[u]), (target, mapping[v]), (key, k)]))
            for u, v, k, d in G.edges(keys=True, data=True)]
    else:
        data[links] = [
            dict(chain(d.items(),
                       [(source, mapping[u]), (target, mapping[v])]))
            for u, v, d in G.edges(data=True)]
    return data
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