Stable Array.prototype.sort

Published · tagged with ECMAScript ES2019

Let’s say you have an array of dogs, where each dog has a name and a rating. (If this sounds like a weird example, you should know that there’s a Twitter account that specializes in exactly this… Don’t ask!)

// Note how the array is pre-sorted alphabetically by `name`.
const doggos = [
{ name: 'Abby', rating: 12 },
{ name: 'Bandit', rating: 13 },
{ name: 'Choco', rating: 14 },
{ name: 'Daisy', rating: 12 },
{ name: 'Elmo', rating: 12 },
{ name: 'Falco', rating: 13 },
{ name: 'Ghost', rating: 14 },
];
// Sort the dogs by `rating` in descending order.
// (This updates `doggos` in place.)
doggos.sort((a, b) => b.rating - a.rating);

The array is pre-sorted alphabetically by name. To sort by rating instead (so we get the highest-rated dogs first), we use Array#sort, passing in a custom callback that compares the ratings. This is the result that you’d probably expect:

[
{ name: 'Choco', rating: 14 },
{ name: 'Ghost', rating: 14 },
{ name: 'Bandit', rating: 13 },
{ name: 'Falco', rating: 13 },
{ name: 'Abby', rating: 12 },
{ name: 'Daisy', rating: 12 },
{ name: 'Elmo', rating: 12 },
]

The dogs are sorted by rating, but within each rating, they’re still sorted alphabetically by name. For example, Choco and Ghost have the same rating of 14, but Choco appears before Ghost in the sort result, because that’s the order they had in the original array as well.

To get this result however, the JavaScript engine can’t just use any sorting algorithm — it has to be a so-called “stable sort”. For a long time, the JavaScript spec didn’t require sort stability for Array#sort, and instead left it up to the implementation. And because this behavior was unspecified, you could also get this sort result, where Ghost now suddenly appears before Choco:

[
{ name: 'Ghost', rating: 14 }, // 😢
{ name: 'Choco', rating: 14 }, // 😢
{ name: 'Bandit', rating: 13 },
{ name: 'Falco', rating: 13 },
{ name: 'Abby', rating: 12 },
{ name: 'Daisy', rating: 12 },
{ name: 'Elmo', rating: 12 },
]

In other words, JavaScript developers could not rely on sort stability. In practice, the situation was even more infuriating, since some JavaScript engines would use a stable sort for short arrays and an unstable sort for larger arrays. This was really confusing, as developers would test their code, see a stable result, but then suddenly get an unstable result in production when the array was slightly bigger.

But there’s some good news. We proposed a spec change that makes Array#sort stable, and it was accepted. All major JavaScript engines now implement a stable Array#sort. It’s just one less thing to worry about as a JavaScript developer. Nice!

(Oh, and we did the same thing for TypedArrays: that sort is now stable as well.)

Note: Although stability is now required per spec, JavaScript engines are still free to implement whatever sorting algorithm they prefer. V8 uses Timsort, for example. The spec does not mandate any particular sorting algorithm.

Feature support

Stable Array.prototype.sort

Stable %TypedArray%.prototype.sort