Concat a collection's sub arrays and merge their parent I have an array like this:

var arr = [
    {
        revision: 19,
        text: 'hello',
        details: [
            {
                id: 5
            }
        ]
    },
    {
        revision: 19,
        text: 'hello',
        details: [
            {
                id: 6
            }
        ]
    },
    {
        revision: 17,
        text: 'world',
        details: [
            {
                id: 7
            }    
        ]
    }
];


And I'd like to concat its details arrays if the revision is the same (the text values are the same if they got the same revision value):

var expected = [
    {
        revision: 19,
        text: 'hello',
        details: [
            {
                id: 5
            },
            {
                id: 6
            }
        ]
    },
    {
        revision: 17,
        text: 'world',
        details: [
            {
                id: 7
            }    
        ]
    }
];


Here is what I came up with:

constructArray(groupByRevisionNb(arr));

function groupByRevisionNb(arr) {
    return _.groupBy(arr, 'revision');
}

function constructArray(obj) {
    return _.map(obj, function (arr) {
        return concatRevisions(arr);
    });
}

function concatRevisions(arr) {
    return _.reduce(arr, function (obj1, obj2) {
        if (_.isEmpty(obj1)) {
            obj1 = _.clone(obj2);
        } else {
            obj1.details = obj1.details.concat(obj2.details);
        }
        return obj1;
    }, {});
}


But I'm not 100% satisfied with it. What could be improved?

I'm not using ES6.
This can be done natively:

var revisionMap = arr.reduce((map, entry) =&gt; {
  var revision = entry.revision;

  if(map.hasOwnProperty(revision))
    map[revision].details.push(...entry.details);
  else
    map[revision] = _.clone(entry);

  return map;
}, {});

/*
{
  1: { revision: REV, text: TEXT, details: [] },
  2: { revision: REV, text: TEXT, details: [] },
}
*/

var expected = Object.keys(revisionMap).map( key =&gt; revisionMap[key] );

/*
[
  { revision: REV, text: TEXT, details: [] },
  { revision: REV, text: TEXT, details: [] },
]
*/


First, make a mapping of ID to revision data. This way, we can easily check the existence of an already visited revision and know when to append rather than assign. Then, we convert that object into an array.

The only thing not native is the use of _.clone as we don't want to be modifying the original data when we're be appending details.