30-seconds-of-code/snippets/stableSort.md

stableSort

Performs stable sort. Useful in Chrome and NodeJS.

Use Array.map() to pair each element of the input array with its corresponding index. Then use Array.sort() and a user provided compare() function. If the items are equal, sort them by their initial index in the input array. Lastly use Array.map() to convert back to the initial array items. Returns new array without modifying the initial one.

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);

const str = 'abcdefghijklmnopqrstuvwxyz';
const compare = (a, b) => ~~(str.indexOf(b) / 2.3) - ~~(str.indexOf(a) / 2.3);

// default Array.sort() is unstable in Chrome and NodeJS + modifies the input array
var arr1 = str.split('');
console.log(arr1.sort(compare).join('') === 'xyzvwtursopqmnklhijfgdeabc'); // false

// stable sort + returns new array
const arr2 = str.split('');
console.log(stableSort(arr2, compare).join('') === 'xyzvwtursopqmnklhijfgdeabc'); // true