---
title: Stable sort
tags: array
cover: horse-sunset
firstSeen: 2018-02-06T18:33:49+02:00
lastUpdated: 2020-10-22T20:24:30+03:00
---

Performs stable sorting of an array, preserving the initial indexes of items when their values are the same.

- Use `Array.prototype.map()` to pair each element of the input array with its corresponding index.
- Use `Array.prototype.sort()` and a `compare` function to sort the list, preserving their initial order if the items compared are equal.
- Use `Array.prototype.map()` to convert back to the initial array items.
- Does not mutate the original array, but returns a new array instead.

```js
const stableSort = (arr, compare) =>
  arr
    .map((item, index) => ({ item, index }))
    .sort((a, b) => compare(a.item, b.item) || a.index - b.index)
    .map(({ item }) => item);
```

```js
const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const stable = stableSort(arr, () => 0); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
```