Underscore: groupBy ignores/destroys sort order?

[jat850]

I have a sorted array, and upon passing it through _.groupBy, it seems as though it returns results out-of-order from what I passed values in.

Example (sorted in descending order by each column):

[ { key: [ 2887902, 2011, 7, 13, 13, 14 ],
value: [ 0.27999999999883585, 0.5, 1 ] },
{ key: [ 2887902, 2011, 7, 13, 7, 15 ],
value: [ 5.149999999994179, 2.5, 1 ] },
{ key: [ 2887902, 2011, 7, 13, 6, 26 ],
value: [ 0, 0.5, 1 ] },
{ key: [ 2820795, 2012, 7, 23, 19, 14 ],
value: [ 17.349999999976717, 6, 1 ] },
{ key: [ 2820795, 2011, 7, 15, 13, 3 ],
value: [ 3.4349999999976717, 3.5, 1 ] },
{ key: [ 2820795, 2011, 7, 15, 12, 55 ],
value: [ 188.02000000001863, 118.5, 1 ] },
{ key: [ 2820795, 2011, 7, 14, 18, 11 ],
value: [ 17.51500000001397, 7.5, 1 ] },
{ key: [ 2820795, 2011, 7, 14, 17, 43 ],
value: [ 9.46999999997206, 5, 1 ] },
{ key: [ 2820795, 2011, 6, 23, 19, 14 ],
value: [ 17.320000000006985, 7, 1 ] } ]

var output = _.groupBy(array, function(out_key) {
return out_key.key[0];
}

My output is in the following order:

[ { key: [ 2820795, 2012, 7, 23, 19, 14 ],
value: [ 17.349999999976717, 6, 1 ] },
{ key: [ 2820795, 2011, 7, 15, 13, 3 ],
value: [ 3.4349999999976717, 3.5, 1 ] },
{ key: [ 2820795, 2011, 7, 15, 12, 55 ],
value: [ 188.02000000001863, 118.5, 1 ] },
{ key: [ 2820795, 2011, 7, 14, 18, 11 ],
value: [ 17.51500000001397, 7.5, 1 ] },
{ key: [ 2820795, 2011, 7, 14, 17, 43 ],
value: [ 9.46999999997206, 5, 1 ] },
{ key: [ 2820795, 2011, 6, 23, 19, 14 ],
value: [ 17.320000000006985, 7, 1 ] } ]

[ { key: [ 2887902, 2011, 7, 13, 13, 14 ],
value: [ 0.27999999999883585, 0.5, 1 ] },
{ key: [ 2887902, 2011, 7, 13, 7, 15 ],
value: [ 5.149999999994179, 2.5, 1 ] },
{ key: [ 2887902, 2011, 7, 13, 6, 26 ],
value: [ 0, 0.5, 1 ] } ]

Is this expected or have I done something wrong? (Within the two result sets, the results are sorted. But I would have expected to see 2887902 first, then 2820795.)

[ascheink]

The underlying issue here is that most engines preserve insertion order except for array indices. See, for example, http://code.google.com/p/v8/issues/detail?id=164

So, in Chrome:

t = {}; t['two'] = 2; t['one'] = 1;
_.keys(t)
> ["two", "one"] // insertion order preserved

t = {}; t[2] = 2; t[1] = 1;
_.keys(t)
> [1, 2] // insertion order not preserved

Since your groupBy function returns integers, they are treated as array indices and insertion order is lost.