How to group nested array object using value

Question

I have a below object where I am trying to group data using department and sub department value. I have tried using reduce function on the department name but could not achieve the desired result.

For example, I have an array of department objects:

var arr = //Actual Object
  [{
      "department": "Admin",
      "sub_department": [{
        "name": "Finance",
        "application": [{
          "name": "F1",
          "tag": 100
        }]
      }]
    },
    {
      "department": "Admin",
      "sub_department": [{
        "name": "Registry",
        "application": [{
          "name": "R2",
          "tag": 200
        }]
      }]
    },
    {
      "department": "Admin",
      "sub_department": [{
        "name": "Finance",
        "application": [{
          "name": "F2",
          "tag": 200
        }]
      }]
    },
    {
      "department": "Helpdesk",
      "sub_department": [{
        "name": "Entry",
        "application": [{
          "name": "E1",
          "tag": 200
        }]
      }]
    }
  ]

//Tried below code
var result = arr.reduce(function(r, a) {
  r[a.department] = r[a.department] || [];
  r[a.department].push(a);
  return r;
}, Object.create(null));

console.log(result)

And I am trying to achieve the desired result by grouping same department and if both department and sub_department.name is same then pushing the application into sub_department array.

[
  {
    "department": "Admin",
    "sub_department": [
      {
        "name": "Finance",
        "application": [
          {
            "name": "F1",
            "tag": 100
          },
          {
            "name": "F2",
            "tag": 200
          }
        ]
      },
      {
        "name": "Registry",
        "application": [
          {
            "name": "R2",
            "tag": 200
          }
        ]
      }
    ]
  },
  {
    "department": "Helpdesk",
    "sub_department": [
      {
        "name": "Entry",
        "application": [
          {
            "name": "E1",
            "price": 200
          }
        ]
      }
    ]
  }
]

Any help on this will be really helpful.

Possible duplicate of Group array of object nesting some of the keys with specific names — Heretic Monkey
– Heretic Monkey, Commented Feb 28, 2018 at 18:36

Jonas Wilms · Accepted Answer · 2018-02-28 18:01:42Z

You were actually quite close. Just build up objects as the hashtables values and use Object.values to turn the hashtable into an array:

var result = Object.values(arr.reduce(function(r, a) { //<--- the hashtable > array conversion
  r[a.department] = r[a.department] || {
     department: a.department,
     sub_department:[]
  }; // <-- objects instead of arrays in the hashtable

  r[a.department].sub_department.push(...a.sub_department); //and add the subdepartment not the object itself
  return r;
 }, Object.create(null)));

console.log(result);

Alternatively you could build up a hashtable and the resulting array in parallel, thats how i would do that:

 const result = [], hash = {};

 for(const {department, sub_department} of arr){
   if(hash[department]){
     hash[department].push(...sub_department);
   } else {
     result.push({department, sub_department});
     hash[department] = sub_department;
  }
}

To also group the sub_departments its slightly more complicated. For that we would need a nested hashtable and a nested lookup:

 const exit = Symbol("exit");
 const result = [], hash = {};

 for(const {department, sub_department} of arr){
   if(!hash[department]){
     const sub = [];
     hash[department] = {[exit]: sub};
     result.push({department, sub_department: sub});
   }

   for(const {name, ...data} of sub_department){
     if(hash[department][name]){
       hash[department][name].push(data);
     } else {
       const subsub = [];
       hash[department][exit].push({name, subsub});
       hash[department][name] = subsub;
     }
  }
}

Sub department grouping is not working as expected with my desired result

Parama Kar · Accepted Answer · 2018-03-01 06:30:26Z

A somewhat long-winded solution extending the previously posted solution:

var result = Object.values(arr.reduce(function(r, a) { //<--- the hashtable > array conversion
    r[a.department] = r[a.department] || {
        department: a.department,
        sub_department:[]
    }; // <-- objects instead of arrays in the hashtable

    if(r[a.department].sub_department.length>0){
        var index = r[a.department].sub_department.findIndex(function(sd){
            return sd.name === a.sub_department[0].name;
        });
        if(index!==-1){
            r[a.department].sub_department[index].application.push(a.sub_department[0].application[0]);
            return r;
        }
    }

    r[a.department].sub_department.push(...a.sub_department); //and add the sub department not the object itself
    return r;


  }, Object.create(null)));
    console.log(result);

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