Array Manipulation: merge objects [ES6]

Question

So i've been struggling for a few hours now. Here's what I'm trying to do.

I have an array of objects:

initArr = [
  {id: 5, time: 100, download: true},
  {id: 2, time: 50, download: false},
  {id: 3, time: 1000, download: true},
  {id: 5, time: 50, download: true},
  {id: 5, time: 550, download: false},
  {id: 2, time: 1500, download: true}
]

some objects have the same id, I want to merge them together.
- but when merging i want to sum the time value of both merged object .
- if one of the two merged objects has download: false, the merged object should have false, otherwise true.

Here's what I have so far (I did not start yet even considering the download key):

const mergedArr= [];

initArr.map(obj => obj['id'])
     .map((id, i, arr) => {
        if (arr.indexOf(id) === i) {
            mergedArr.push(initArr[i]);
        } else { 
          const mergeIndex = mergedArr.map( x => x.id).indexOf(id);
          mergedArr[mergeIndex].playTime +=initArr[arr.indexOf(id)].playTime;
        }
    return mergedArr
});

I'd love so inputs or hints :)

Is preserving the order important?

user234932
– user234932

2019-07-16 14:24:51 +00:00
Commented Jul 16, 2019 at 14:24 — user234932
– user234932, Commented Jul 16, 2019 at 14:24
yes ! I should have mentioned that :)

estellechvl
– estellechvl

2019-07-16 14:55:34 +00:00
Commented Jul 16, 2019 at 14:55 — estellechvl
– estellechvl, Commented Jul 16, 2019 at 14:55

adiga · Accepted Answer · 2019-07-16 14:38:39Z

2

You could reduce the array. Create an accumulator object with each id as key and the merged object as value. If the id already exists, update time and download with current object. Else, add the current id as key and set a copy of the current object as it's value. Then use Object.values() to get an array of values from this accumulator object.

const initArr = [
  { id: 5, time: 100, download: true },
  { id: 2, time: 50, download: false },
  { id: 3, time: 1000, download: true },
  { id: 5, time: 50, download: true },
  { id: 5, time: 550, download: false },
  { id: 2, time: 1500, download: true }
]

const merged = initArr.reduce((acc, o) => {
  if (acc[o.id]) {
    acc[o.id].time += o.time;
    acc[o.id].download = acc[o.id].download && o.download;
  } else
    acc[o.id] = { ...o };

  return acc;
}, {})

console.log(Object.values(merged))

edited Jul 16, 2019 at 14:38

answered Jul 16, 2019 at 14:24

adiga

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8 Comments

Ori Drori Over a year ago

This case is not fit for an object accumulator, since the ids are numeric. You can see that the order has changed (id: 2 is the 1st item).

Kobe Over a year ago

Can't you just use r[o.id] = o ?

adiga Over a year ago

@Kobe If not cloned, when r[o.id].time += o.time is done, it will mutate the original array's object

Bergi Over a year ago

@OriDrori Why would an object not fit here? It works as well as an array or a Map. Order in the output is hardly relevant.

estellechvl Over a year ago

I should have specified indeed that the order matter here ! but thanks a lot for the detailed explanation !

|

Ori Drori · Accepted Answer · 2019-07-16 15:20:02Z

1

Reduce the array to a Map by the object's id, and then convert back to array using Map.values() and Array.from() (or by spreading into an array).

Note: in this case using Map as an accumulator is better than using an object, since the id property is numeric. In ES6 objects usually maintain the order of insertion, unless the properties are numeric. When an object with numeric property keys is convert to an array, the numeric properties come first, and they are ordered by their value. On the other hand, a Map always maintains the order of insertion.

const initArr = [
  {id: 5, time: 100, download: true},
  {id: 2, time: 50, download: false},
  {id: 3, time: 1000, download: true},
  {id: 5, time: 50, download: true},
  {id: 5, time: 550, download: false},
  {id: 2, time: 1500, download: true}
]

const result = Array.from(initArr.reduce((r, o) => {
  if(!r.has(o.id)) r.set(o.id, { ...o });
  else {
    const current = r.get(o.id);
    current.time += o.time;
    current.download = current.download && o.download
  }
  
  return r;
}, new Map).values());

console.log(result);

edited Jul 16, 2019 at 15:20

answered Jul 16, 2019 at 14:22

Ori Drori

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2 Comments

estellechvl Over a year ago

+1 thanks !! With the order being important in my case, this solution is very helpful. I'm not sure I quite understand it completely yet. I mean I get the reduce part. I've never used new Map though. I'll try to read on that. thanks again

Ori Drori Over a year ago

You're welcome :) I'll add a link to info about Map as well.

prabhatojha · Accepted Answer · 2019-07-16 14:39:04Z

1

Try this, supports download option as well

const arr = [
  {id: 5, time: 100, download: true},
  {id: 2, time: 50, download: false},
  {id: 3, time: 1000, download: true},
  {id: 5, time: 50, download: true},
  {id: 5, time: 550, download: false},
  {id: 2, time: 1500, download: true}
]

const result = arr.reduce((res, obj) => {
    const found = res.find(t => t.id === obj.id);

    if(found){
        found.time += obj.time;
        found.download = found.download && obj.download;    
    } else {
        res.push(obj);
    }
    return res;
}, []);

console.log(result);

answered Jul 16, 2019 at 14:39

prabhatojha

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Comments

Kobe · Accepted Answer · 2019-07-16 14:26:19Z

0

You can use reduce to:

check if the array has the item with the ID on each loop
if it does, add the time

else

push the object

const arr = [
  {id: 5, time: 100, download: true},
  {id: 2, time: 50, download: false},
  {id: 3, time: 1000, download: true},
  {id: 5, time: 50, download: true},
  {id: 5, time: 550, download: false},
  {id: 2, time: 1500, download: true}
]

console.log(arr.reduce((a, o) => {
  const i = a.findIndex(({id}) => id === o.id)
  i + 1 ? a[i].time += o.time : a.push(o)
  return a
}, []))

edited Jul 16, 2019 at 14:26

answered Jul 16, 2019 at 14:20

Kobe

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Comments

ttulka · Accepted Answer · 2019-07-16 14:38:05Z

0

One more reduce solution:

const initArr = [
  {id: 5, time: 100, download: true},
  {id: 2, time: 50, download: false},
  {id: 3, time: 1000, download: true},
  {id: 5, time: 50, download: true},
  {id: 5, time: 550, download: false},
  {id: 2, time: 1500, download: true}
]

const merged = initArr.reduce((a,c) => {
    const found = a.find(x => x.id === c.id);
    if (found) {
        found.time += c.time;
        found.download = found.download && c.download;
        return a;
    }
    return [...a, {...c}];
}, []);

console.log(merged);

answered Jul 16, 2019 at 14:38

ttulka

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Comments

loki97 · Accepted Answer · 2019-07-18 05:12:36Z

0

        function manipulation(initArr){
            var resultArray = [];
            var arrMap = new Map();
            var currIndex = 0;
            initArr.forEach(function(item, index){
                if(arrMap.has(item.id)){
                    var existingItem = resultArray[arrMap.get(item.id)];
                    existingItem.time = item.time+existingItem.time;
                    if(existingItem.download && !item.download){
                        existingItem.download = false;
                    }
                }else{
                    arrMap.set(item.id, currIndex);
                    resultArray.push(item);
                    currIndex++;
                }
            })
            return resultArray;
        }

edited Jul 18, 2019 at 5:12

answered Jul 16, 2019 at 14:49

loki97

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