lodash sortBy then groupBy, is order maintained?

Question

I'm having trouble figuring out from the lodash documentation if my assumption about sorting and grouping is correct.

If I use sortBy, then use groupBy, do the arrays produced by groupBy maintain the sort order of items?

For example, say I have the following array:

var testArray = [[5,6],[1,3],[5,4],[5,1]]

And I would like to group these by their first element, but also have them sorted by their second element within these groups. So, in lodash I assume I can do the following:

_.chain(testArray)
  .sortBy(function (item) { return item[1]; })
  .groupBy(function (item) { return item[0]; })
  .value()

Which ends up producing what I would expect it to:

{
  1: [[1,3]]
  5: [[5,1],[5,4],[5,6]]
}

Is this just coincidence? Is there anything about how sortBy and groupBy work that ensures this ordering of the grouped arrays? The documentation says that sortBy is a stable sort, does that in the same way apply to groupBy? Is there any reason I should not assume this will work every time?

score 16 · Answer 1 · answered Dec 09 '16 at 12:54

16

It's not. Here's example, where order is not retained:

const data = [
  {
    item: 'item1',
    group: 'g2'
  },   {
    item: 'item2',
    group: 'g3'
  },   {
    item: 'item3',
    group: 'g1'
  },   {
    item: 'item4',
    group: 'g2'
  },   {
    item: 'item5',
    group: 'g3'
  }
]

const groupedItems = _(data).groupBy(item => item.group).value()

In this case one would expect that group order would be: g2, g3, g1 - reality is that they are sorted g1, g2, g3.

You can re-sort them with original array though.

const groupedItems = _(data)
  .groupBy(item => item.group)
  .sortBy(group => data.indexOf(group[0]))
  .value()

This will ensure original order of items.

answered Dec 09 '16 at 12:54

Hatch

558
5
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3

This is pure gold: `.sortBy(group => data.indexOf(group[0]))` . Saved me an hour in trying to get the damn thing working! – Boris Marinov Feb 14 '19 at 08:34
1

Maybe there was a change to the library, but your first examples output comes out in the group order expected. I'm using lodash v4.17.15 – philip yoo Aug 19 '19 at 19:12
I don't think that's what the question is about. Of course the groups are not sorted, because object properties are not sorted (the `groupBy` gives an object, not an array!). The question is about the order inside each group, not about the order of the groups. – Ella Sharakanski May 27 '20 at 10:40

score 11 · Accepted Answer · answered Mar 04 '15 at 17:20

The current implementation of _.groupBy is:

// An internal function used for aggregate "group by" operations.
var group = function(behavior) {
  return function(obj, iteratee, context) {
    var result = {};
    iteratee = cb(iteratee, context);
    _.each(obj, function(value, index) {
      var key = iteratee(value, index, obj);
      behavior(result, value, key);
    });
    return result;
  };
};

// Groups the object's values by a criterion. Pass either a string attribute
// to group by, or a function that returns the criterion.
_.groupBy = group(function(result, value, key) {
  if (_.has(result, key)) result[key].push(value); else result[key] = [value];
});

Basically it iterates through each of the items in the collection in order (if the collection is array-like, which it would be after a sortBy), and pushes them to an array based on their key value.

So yes, I'm not sure if this is an "official" characteristic of _.groupBy, but it does preserve the order of array-like collections, and that's probably unlikely to change.

So the sub-items within groups would retain their original ordering, but the grouped key ordering may change, because they are object properties. — Sam Barnum, Apr 05 '16 at 00:12

score 1 · Answer 3 · edited May 23 '17 at 12:00

1

Function groupBy returns object. Object doesn't save property order. Does JavaScript Guarantee Object Property Order?

But group arrays saves order, because thay are added with push function.

edited May 23 '17 at 12:00

Community

1
1

answered Mar 09 '16 at 20:21

Denis535

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30

My question was about the ordering of the resulting arrays, not the parent object produced by `groupBy`. – kand Mar 09 '16 at 21:31
Update a little my answer. Just I had trouble with object property order and I decided to write this. – Denis535 Mar 10 '16 at 09:39

lodash sortBy then groupBy, is order maintained?

3 Answers3

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