D3.js Tree - Paging For Many Vertical Child Nodes?

I'm finding that the number of child nodes in a d3.js tree need to be constrained to prevent tight clustering of many nodes. An extreme example: I have been attempting to do this

Solution 1:

var treeData = {
  "name": "Top Level",
  "children": [{
      "name": "Child 1",
      "children": [{
          "name": "C1"
        },
        {
          "name": "C2"
        },
        {
          "name": "C3"
        },
        {
          "name": "C4"
        },
        {
          "name": "C5"
        },
        {
          "name": "C6"
        },
        {
          "name": "C7"
        },
        {
          "name": "C8"
        },
        {
          "name": "C9"
        },
        {
          "name": "C10"
        },
        {
          "name": "C11"
        },
        {
          "name": "C12"
        },
        {
          "name": "C13"
        },
        {
          "name": "C14"
        },
        {
          "name": "C15"
        },
        {
          "name": "C16"
        },
        {
          "name": "C17"
        },
        {
          "name": "C18"
        },
        {
          "name": "C19"
        },
        {
          "name": "C20"
        },
        {
          "name": "C21"
        },
      ]
    },
    {
      "name": "Child 2",
      "children": [{
          "name": "Son of Child 2"
        },
        {
          "name": "Daughter of Child 2"
        }
      ]
    }
  ]
};

// Set the dimensions and margins of the diagram
var margin = {
    top: 20,
    right: 90,
    bottom: 30,
    left: 90
  },
  width = 960 - margin.left - margin.right,
  height = 500 - margin.top - margin.bottom;

// append the svg object to the body of the page
// appends a 'group' element to 'svg'
// moves the 'group' element to the top left margin
var svg = d3.select("body").append("svg")
  .attr("width", width + margin.right + margin.left)
  .attr("height", height + margin.top + margin.bottom)
  .append("g")
  .attr("transform", "translate(" +
    margin.left + "," + margin.top + ")");

var i = 0,
  duration = 750,
  root;

// declares a tree layout and assigns the size
var treemap = d3.tree().size([height, width]);

// Assigns parent, children, height, depth
root = d3.hierarchy(treeData, function(d) {
  return d.children;
});
root.x0 = height / 2;
root.y0 = 0;

function pageNodes(d, maxNode) {
  if (d.children) {
    d.children.forEach(c => pageNodes(c, maxNode));
    if (d.children.length > maxNode) {
      d.pages = {}
      const count = maxNode - 2;
      const l = Math.ceil(d.children.length / count);
      for (let i = 0; i < l; i++) {
        let startRange = i * count;
        let endRange = i * count + count;
        d.pages[i] = d.children.slice(startRange, endRange);
        d.pages[i].unshift({
          ...d.pages[i][0],
          data: {
            name: "..."
          },
          page: i == 0 ? l - 1 : i - 1
        })

        // console.log(i, d.pages[i]);
        d.pages[i].push({
          ...d.pages[i][0],
          data: {
            name: "..."
          },
          page: i != (l - 1) ? i + 1 : 0,
        });
      }
      d.children = d.pages[0];
    }
  }
}

root.children.forEach(c => pageNodes(c, 10));

// Collapse after the second level
root.children.forEach(collapse);

update(root);

// Collapse the node and all it's children
function collapse(d) {
  if (d.children) {
    d._children = d.children
    d._children.forEach(collapse)
    d.children = null
  }
}


function update(source) {

  const colourScale = d3.scaleOrdinal()
    .domain(["Parent", "Child 1", "Child 2"])
    .range(["#abacab", "#b67a4e", "#5a6fbb"]);

  // Assigns the x and y position for the nodes
  var treeData = treemap(root);

  // Compute the new tree layout.
  var nodes = treeData.descendants(),
    links = treeData.descendants().slice(1);

  // Normalize for fixed-depth.
  nodes.forEach(function(d) {
    d.y = d.depth * 180
  });

  // ****************** Nodes section ***************************

  // Update the nodes...
  var node = svg.selectAll('g.node')
    .data(nodes, function(d) {
      return d.id || (d.id = ++i);
    });

  // Enter any new modes at the parent's previous position.
  var nodeEnter = node.enter().append('g')
    .attr('class', 'node')
    .attr("transform", function(d) {
      return "translate(" + source.y0 + "," + source.x0 + ")";
    })
    .on('click', click);

  // Add Circle for the nodes
  nodeEnter.append('circle')
    .attr('class', 'node')
    .attr('r', 1e-6)
    .style("fill", function(d) {
      return colourScale(findParent(d))
    });

  // Add labels for the nodes
  nodeEnter.append('text')
    .attr("dy", ".35em")
    .attr("x", function(d) {
      return d.children || d._children ? -13 : 13;
    })
    .attr("text-anchor", function(d) {
      return d.children || d._children ? "end" : "start";
    })
    .text(function(d) {
      return d.data.name;
    });

  // UPDATE
  var nodeUpdate = nodeEnter.merge(node);

  // Transition to the proper position for the node
  nodeUpdate.transition()
    .duration(duration)
    .attr("transform", function(d) {
      return "translate(" + d.y + "," + d.x + ")";
    });

  // Update the node attributes and style
  nodeUpdate.select('circle.node')
    .attr('r', 6)
    .style("fill", function(d) {
      return colourScale(findParent(d));
    });


  // Remove any exiting nodes
  var nodeExit = node.exit().transition()
    .duration(duration)
    .attr("transform", function(d) {
      return "translate(" + source.y + "," + source.x + ")";
    })
    .remove();

  // On exit reduce the node circles size to 0
  nodeExit.select('circle')
    .attr('r', 1e-6);

  // On exit reduce the opacity of text labels
  nodeExit.select('text')
    .style('fill-opacity', 1e-6);

  // ****************** links section ***************************

  // Update the links...
  var link = svg.selectAll('path.link')
    .data(links, function(d) {
      return d.id;
    });

  // Enter any new links at the parent's previous position.
  var linkEnter = link.enter().insert('path', "g")
    .attr("class", "link")
    .attr("stroke-width", function(d) {
      return 1;
    })
    .attr('d', function(d) {
      var o = {
        x: source.x0,
        y: source.y0
      }
      return diagonal(o, o)
    })
    .attr("opacity", "0.3")
    .style("stroke", function(d) {
      //return colourScale(findParentLinks(d));
    });

  // UPDATE
  var linkUpdate = linkEnter.merge(link);

  // Transition back to the parent element position
  linkUpdate.transition()
    .duration(duration)
    .attr('d', function(d) {
      return diagonal(d, d.parent)
    });

  // Remove any exiting links
  var linkExit = link.exit().transition()
    .duration(duration)
    .attr('d', function(d) {
      var o = {
        x: source.x,
        y: source.y
      }
      return diagonal(o, o)
    })
    .remove();

  // Store the old positions for transition.
  nodes.forEach(function(d) {
    d.x0 = d.x;
    d.y0 = d.y;
  });

  // Creates a curved (diagonal) path from parent to the child nodes
  function diagonal(s, d) {

    path = `M ${s.y} ${s.x}
            C ${(s.y + d.y) / 2} ${s.x},
              ${(s.y + d.y) / 2} ${d.x},
              ${d.y} ${d.x}`

    return path
  }

  function findParent(datum) {
    if (datum.depth < 2) {
      return datum.data.name
    } else {
      return findParent(datum.parent)
    }
  }

  function findParentLinks(datum) {
    if (datum.target.depth < 2) {
      return datum.target.name
    } else {
      return findParent(datum.target.parent)
    }
  }

  // Toggle children on click.
  function click(event, d) {
    if (d.hasOwnProperty('page')) {
      d.parent.children = d.parent.pages[d.page];
    } else if (d.children) {
      d._children = d.children;
      d.children = null;
    } else {
      d.children = d._children;
      d._children = null;
    }
    update(d);
  }
}

Here working example