Group by array of objects by a nested key

Question

I have the following data:

const data = [
    {
        parent: {
            id: "1",
            name: "Europe"
        },
        item: {
            name: "Italy"
        },
        score: 5
    },
    {
        parent: {
            id: "1",
            name: "Europe"
        },
        item: {
            name: "France"
        },
        score: 4.5
    },
    {
        parent: {
            id: "1",
            name: "Europe"
        },
        item: {
            name: "UK"
        },
        score: 4.9
    },
    {
        parent: {
            id: "2",
            name: "Afrique"
        },
        item: {
            name: "Morocco"
        },
        score: 3.1
    },
    {
        parent: {
            id: "2",
            name: "Afrique"
        },
        item: {
            name: "Egypt"
        },
        score: 3.9
    }
];

I want to group it based on the parent.id and calculate the average score, so I can have the following result:

[
    {
        parent: {
            id: "1",
            name: "Europe",
            items: [
                {
                    name: "Italy"
                },
                {
                    name: "France"
                },
                {
                    name: "UK"
                }
            ],
            score: 4.8
        }
    },
    {
        parent: {
            id: "2",
            name: "Afrique",
            items: [
                {
                    name: "Morocco"
                },
                {
                    name: "Egypt"
                }
            ],
            score: 3.5
        }
    }
]

I used the following function, but it doesn't work for the nested key, and also it's doesn't return the desired result schema.

let group = cars.reduce((r, a) => {
   console.log("a", a);
   console.log('r', r);
   r[a.make] = [...r[a.parent.id] || [], a];
   return r;
}, {});
console.log("group", group);

What is a.make? There's no make property in your objects. — Barmar
– Barmar, Commented Mar 31, 2020 at 22:26
Shouldn't the resulting array, have the scores inside each item, not as an attribute of the parent? — Victor
– Victor, Commented Mar 31, 2020 at 22:44
@GrandPhuba yea, actually that's better, I'll update the question. THANKS — Ayoub k
– Ayoub k, Commented Mar 31, 2020 at 22:46

yyc · Accepted Answer · 2020-03-31 23:58:04Z

You can use _reduce() function: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce

var result = data.reduce((res, data) => {
  if(!res[data.parent.id]) {
    data.item = [data.item];
    res[data.parent.id] = data;
  } else {
    res[data.parent.id]['item'].push(data['item']);
    res[data.parent.id]['score'] = (res[data.parent.id]['score'] + data['score'])/2;
  }
  return res;
}, [])
.filter(x => x != null)

const data = [
    {
        parent: {
            id: "1",
            name: "Europe"
        },
        item: {
            name: "Italy"
        },
        score: 5
    },
    {
        parent: {
            id: "1",
            name: "Europe"
        },
        item: {
            name: "France"
        },
        score: 4.5
    },
    {
        parent: {
            id: "1",
            name: "Europe"
        },
        item: {
            name: "UK"
        },
        score: 4.9
    },
    {
        parent: {
            id: "2",
            name: "Afrique"
        },
        item: {
            name: "Morocco"
        },
        score: 3.1
    },
    {
        parent: {
            id: "2",
            name: "Afrique"
        },
        item: {
            name: "Egypt"
        },
        score: 3.9
    }
];

var result = data.reduce((res, data) => {
  if(!res[data.parent.id]) {
    data.item = [data.item];
    res[data.parent.id] = data;
  } else {
res[data.parent.id]['item'].push(data['item']);
res[data.parent.id]['score'] = (res[data.parent.id]['score'] + data['score'])/2;
  }
  return res;
}, [])
.filter(x => x != null)

console.log(result)

Victor · Accepted Answer · 2020-03-31 23:07:08Z

Create an object/hashmap, then format the resulting object into an array.

let continents = {}
data.forEach(function(country){
    const continent_id = country.parent.id
    let continent = continents[continent_id]
    if(!continent){
        continent = {
             id: continent_id,
             name: country.parent.name,
             items: [],
        }
        continents[continent_id] = continent
    }
    continent.items.push({
       name: country.item.name,
       score: country.score
    })

})
continents = Object.entries(continents).map(item => ({parent: item[1]}))
console.log(continents)

Output:

[
    {
        "parent":{
            "id":"1",
            "name":"Europe",
            "items":[
                {
                    "name":"Italy",
                    "score":5
                },
                {
                    "name":"France",
                    "score":4.5
                },
                {
                    "name":"UK",
                    "score":4.9
                }
            ]
        }
    },
    {
        "parent":{
            "id":"2",
            "name":"Afrique",
            "items":[
                {
                    "name":"Morocco",
                    "score":3.1
                },
                {
                    "name":"Egypt",
                    "score":3.9
                }
            ]
        }
    }
]

Note, I put the score inside each item as mentioned in the comments of your question

elvira.genkel · Accepted Answer · 2020-03-31 23:47:56Z

From the data you've provided if you additionaly need to count average of score property, use the following reduce method: it will iterate trough your data, group it and calculate total score value and count of score values. And after reduce groups object perform map that will calculate average for score for all the groups using totalScore and scoreCount

const data = [
    {
        parent: {
            id: "1",
            name: "Europe"
        },
        item: {
            name: "Italy"
        },
        score: 5
    },
    {
        parent: {
            id: "1",
            name: "Europe"
        },
        item: {
            name: "France"
        },
        score: 4.5
    },
    {
        parent: {
            id: "1",
            name: "Europe"
        },
        item: {
            name: "UK"
        },
        score: 4.9
    },
    {
        parent: {
            id: "2",
            name: "Afrique"
        },
        item: {
            name: "Morocco"
        },
        score: 3.1
    },
    {
        parent: {
            id: "2",
            name: "Afrique"
        },
        item: {
            name: "Egypt"
        },
        score: 3.9
    }
];

let group = data.reduce((acc, rec) => {
   if (acc.find(item => item.parent.id === rec.parent.id))
     {
       const idx = acc.findIndex(item => item.parent.id === rec.parent.id)
       acc[idx].parent.items = acc[idx].parent.items.concat(rec.item)
       acc[idx].parent.score += rec.score
       acc[idx].parent.scoreCount +=1
       
     } else {
       acc = acc.concat({parent: {...rec.parent, score: rec.score, items: [rec.item], scoreCount:1}})
       
     }
   
   return acc
}, []).map(it => ({parent: {id: it.parent.id, name:it.parent.name, score: (it.parent.score / it.parent.scoreCount), items: it.parent.items}}));

console.log("group", group);

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