Does .sort always keep the chronological order of items?

Question

Take:

let array = [ ['whatever', 3], ['1st', 1], ['2nd', 1] ]

Now if I sort it using:

array.sort((a,b) => a[1] - b[1])

Will I always end up with:

[ [ '1st', 1 ], [ '2nd', 1 ], [ 'whatever', 3 ] ]

Meaning, is 1st encounter of value of 1 always guaranteed to end up before 2nd encounter which would itself end up before 3rd encounter etc?

pwilcox · Accepted Answer · 2019-08-19T18:14:32.733

1

What you're after is a 'stable' sort, and no, Array.prototype.sort does not support this. Good news though, it's just around the corner in ECMAScript 2020. See the introduction here. Namely:

Other updates include requiring that Array.prototype.sort be a stable sort

As for how long it will take to implement in node.js after specification is released, I cannot find any documentation to that effect.

Edit: Even more good news.

Thank you @d9ngle for mentioning https://v8.dev/blog/array-sort. This shows that the v8 engine, as of version 7, supports stable sorting. Node.js runs on v8. So as long as your version of node is up to date enough, Array.prototype.sort will be stable.

To find out which v8 version you are running, see here.

edited Aug 19 '19 at 18:14

answered Aug 19 '19 at 15:06

pwilcox

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have you seen this? https://v8.dev/blog/array-sort it says that node supports stable sort in `V8 v7.0` – d9ngle Aug 19 '19 at 16:55
Thanks @d9ngle. I added this to the answer. – pwilcox Aug 19 '19 at 18:16

score 0 · Answer 2 · answered Aug 19 '19 at 14:45

If compareFunction(a, b) returns 0, leave a and b unchanged with respect to each other, but sorted with respect to all different elements. Note: the ECMAscript standard does not guarantee this behavior, thus, not all browsers respect this.

The answer is: it depends on where you are running the code.

See Array sort description

score 0 · Answer 3 · answered Aug 19 '19 at 15:19

Yes, Array sort() method always keep the first encountered value before the second encountered value.

you can run this snippet on all browsers

https://jsbin.com/pesuwed/edit?js,console

var items = [
  { name: 'Edward', value: 21 },
  { name: 'Sharpe', value: 37 },
  { name: 'And', value: 45 },
  { name: 'The', value: -12 },
  { name: 'Magnetic', value: 13 },
  { name: 'Zeros', value: 37 },
  { name: 'Sharp', value: 37 },
];

var items1 = [
  ['Edward', 21],
  ['Sharpe', 37],
  ['And', 45],
  ['The', -12],
  ['Magnetic', 13],
  ['Zeros', 37],
  ['Sharp', 37],
];

// sort by value
var sortedArray = items.sort(function (a, b) {
  return a.value - b.value;
});

var sortedArray1 = items1.sort(function (a, b) {
  return a[1] - b[1];
});

console.log(sortedArray);
console.log(sortedArray1);

There is a big difference between data outputting as expected and data being GUARANTEED to be output as expected. To address the latter, you have to consult source documentation. — pwilcox, Aug 19 '19 at 17:56

Does .sort always keep the chronological order of items?

3 Answers3