Group by, and sum, and generate an object for each array in JavaScript

Question

I need to group by id and sum, but I need a new object for each result:

let data = [
    {"id":"2018", "name":"test", "total":1200},
    {"id":"2019", "name":"wath", "total":1500},
    {"id":"2019", "name":"wath", "total":1800},
    {"id":"2020", "name":"zooi", "total":1000},
]

I have this code that returns just one object with the result

let result = data.reduce(function (r, o) {
    (r[o.id])?
        r[o.id] += o.total:
        r[o.id] = o.total;
    return r;
});

But I need some like this:

[
    {"id":"2018", "name":"test", "total":1200},
    {"id":"2019", "name":"wath", "total":2300},
    {"id":"2020", "name":"zooi", "total":1000},
]

How can I do it?

Zohaib Ijaz · Accepted Answer · 2018-05-14 20:05:53Z

10

let data =[
    {"id":"2018", "name":"test", "total":1200},
    {"id":"2019", "name":"wath", "total":1500},
    {"id":"2019", "name":"wath", "total":1800},
    {"id":"2020", "name":"zooi", "total":1000},
];

let map = data.reduce((prev, next) =>{
  if (next.id in prev) {
    prev[next.id].total += next.total;
  } else {
     prev[next.id] = next;
  }
  return prev;
}, {});

let result = Object.keys(map).map(id => map[id]);

console.log(result);

answered May 14, 2018 at 20:05

Zohaib Ijaz

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4 Comments

ibrahim mahrir Over a year ago

I don't see why you changed the code of the reduce. The only thing that mattered was Object.keys(map).map(id => map[id])

Zohaib Ijaz Over a year ago

@ibrahimmahrir I did not understand what do you want to say. Is is bad?

ibrahim mahrir Over a year ago

@ZohaibIjaz No no. I meant to say that the reduce code change was irrelevant. Your answer could've been as short as saying, Do result = Object.keys(map).map(id => map[id]);, without having to rewrite reduce code.

Zohaib Ijaz Over a year ago

@ibrahimmahrir No,that will not be correct answer. We need to group by id and sum total values. So reduce is required

aravindanve · Accepted Answer · 2018-05-14 20:15:16Z

5

You can try this:

const result = Object.values(data.reduce((r, o) => (r[o.id]
  ? (r[o.id].total += o.total)
  : (r[o.id] = {...o}), r), {}));

answered May 14, 2018 at 20:15

aravindanve

1,0798 silver badges15 bronze badges

2 Comments

BreadBoard.ini Over a year ago

what does the '?' and the ':' mean?

aravindanve Over a year ago

@BreadBoard.ini it's known as the ternary operator. Basically it's an inline if-else condition. You can read more about it here developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

ibrahim mahrir · Accepted Answer · 2018-05-14 20:10:08Z

Change your reduce to:

let result = data.reduce(function(acc, obj) {             // for each obj in data
    if(acc.map.hasOwnProperty(obj.id)) {                  // if the map contains an object for the current object's id
        acc.map[obj.id].total += +obj.total;              // then add the current object's total to it
    } else {                                              // otherwise
        var newObj = Object.assign({}, obj);              // create a new object (a copy of the current object)
        acc.map[obj.id] = newObj;                         // add the new object to both the map
        acc.data.push(newObj);                            // ... and the data array
    }
    return acc;
}, {data: [], map: {}}).data;                             // the accumulator should have an array for the data objects (which will be our result) and a map object which maps the ids to the objects in the data array. Once our reduce finishes its work, we assign the data array of the accumulator to result

Example:

let data =[ {"id":"2018", "name":"test", "total":1200}, {"id":"2019", "name":"wath", "total":1500}, {"id":"2019", "name":"wath", "total":1800}, {"id":"2020", "name":"zooi", "total":1000} ];

let result = data.reduce(function(acc, obj) {
    if(acc.map.hasOwnProperty(obj.id)) {
        acc.map[obj.id].total += +obj.total;
    } else {
        var newObj = Object.assign({}, obj);
        acc.map[obj.id] = newObj;
        acc.data.push(newObj);
    }
    return acc;
}, {data: [], map: {}}).data;

console.log(result);

This really helped me thanks :) Very simple and detailed example

user9760669 · Accepted Answer · 2018-05-14 20:32:32Z

-1

import uniqBy from 'lodash/uniqBy'

export function transform (array) {
  const newArray = []
  const uniq = uniqBy(array, 'id')

  uniq.forEach(item => {
    const total = array
      .filter(({id}) => item.id === id)
      .reduce((sum, current) => sum + current.total, 0)

    newArray.push({
      ...item,
      total
    })
  })

  return newArray
}

answered May 14, 2018 at 20:32

user9760669

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