Question

86

lodash sortBy then groupBy, is order maintained?

rated 0 times [ 93] [ 7] / answers: 1 / hits: 29858 / 9 Years ago, wed, march 4, 2015, 12:00:00

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?

Answers

Only authorized users can answer the question. Please sign in first, or register a free account.

cayden

Add To Favorites

Follow

Total Points: 314

Total Questions: 107

Total Answers: 101

Location: Slovenia

Member since Wed, Apr 6, 2022

2 Years ago

answered 9 Years ago albert · Accepted Answer

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.