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snippets/js/s/stable-sort.md

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+---
+title: Stable sort
+type: snippet
+language: javascript
+tags: [array]
+cover: horse-sunset
+dateModified: 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]
+```