Hierarchical Tree from Structured Array of Dependent Elements

Question

I am looking for an efficient approach to build a tree from the following data structure in JavaScript. Each entry always has length 5 and there never exists any gaps (i.e. nulls) between the entries.

var geographies = [
    [ 'Denmark', 'Midtjylland', null, null, null ],
    [ 'Denmark', 'Syddanmark', 'Langeland', null, null ],
    [ 'Denmark', 'Syddanmark', 'Ærø', null, null ],
    [ 'Japan', 'Okinawa', 'Izenajima', null, null ],
    [ 'Japan', 'Hokkaido', 'Rishiri', 'Rishiri-to', null ]
]

The desired output should look like this:

[{
    label: "Denmark",
    children: [{
            label: "Midtjylland",
        },
        {
            label: "Syddanmark",
            children: [{
                label: "Langeland"
            },
            {
                label: "Ærø"
            }]
        }
    ]
}, {
    label: "Japan",
    children: [{
        label: "Okinawa",
        children: [{
            label: "Izenajima"
        }]
    }, {
        label: "Hokkaido",
        children: [{
            label: "Rishiri",
            children: [{
                label: 'Rishiri-to'
            }]
        }]
    }]
}]

Answer 1

You can group the arrays on the first element of each subarray, and then call to_tree to the grouped arrays if they are not empty:

function to_tree(d){
   var c = {}
   for (var i of d){
       var k = i.shift();
       c[k] = k in c ? [...c[k], i] : [i]
   }
   return Object.keys(c).map(function(x){return {label:x, ...(c[x].filter(j => j.length).length ? {children:to_tree(c[x].filter(j => j.length))} : {})}})
}
var geographies = [[ 'Denmark', 'Midtjylland', null, null, null ], [ 'Denmark', 'Syddanmark', 'Langeland', null, null ], [ 'Denmark', 'Syddanmark', 'Ærø', null, null ], [ 'Japan', 'Okinawa', 'Izenajima', null, null ], [ 'Japan', 'Hokkaido', 'Rishiri', 'Rishiri-to', null ]]
var geographies1 = [[ 'Greece', null, null, null, null ], [ 'Greece', 'Ionian Islands', 'Lefkada', null, null ], [ 'Greece', 'Attica', 'Salamis', null, null ], [ 'Greece', 'Ionian Islands', 'Cephalonia', null, null ], [ 'Greece', 'Thessaly', 'Skiathos', null, null ], [ 'Greece', 'South Aegean', 'Kea', null, null ], [ 'Greece', 'Attica', 'Kythira', null, null ]]
var r = to_tree(geographies.map(x => x.filter(y => y != null)));
var r1 = to_tree(geographies1.map(x => x.filter(y => y != null)));
console.log(r)
console.log(r1)

Output:

[
  {
    "label": "Denmark",
    "children": [
      {
        "label": "Midtjylland"
      },
      {
        "label": "Syddanmark",
        "children": [
          {
            "label": "Langeland"
          },
          {
            "label": "Ærø"
          }
        ]
      }
    ]
  },
  {
    "label": "Japan",
    "children": [
      {
        "label": "Okinawa",
        "children": [
          {
            "label": "Izenajima"
          }
        ]
      },
      {
        "label": "Hokkaido",
        "children": [
          {
            "label": "Rishiri",
            "children": [
              {
                "label": "Rishiri-to"
              }
            ]
          }
        ]
      }
    ]
  }
]

[
  {
    "label": "Greece",
    "children": [
      {
        "label": "Ionian Islands",
        "children": [
          {
            "label": "Lefkada"
          },
          {
            "label": "Cephalonia"
          }
        ]
      },
      {
        "label": "Attica",
        "children": [
          {
            "label": "Salamis"
          },
          {
            "label": "Kythira"
          }
        ]
      },
      {
        "label": "Thessaly",
        "children": [
          {
            "label": "Skiathos"
          }
        ]
      },
      {
        "label": "South Aegean",
        "children": [
          {
            "label": "Kea"
          }
        ]
      }
    ]
  }
]