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);

// sorted by weight
const input = [
  { height: 100, weight: 80 },
  { height: 90, weight: 90 },
  { height: 70, weight: 95 },
  { height: 100, weight: 100 },
  { height: 80, weight: 110 },
  { height: 110, weight: 115 },
  { height: 100, weight: 120 },
  { height: 70, weight: 125 },
  { height: 70, weight: 130 },
  { height: 100, weight: 135 },
  { height: 75, weight: 140 },
  { height: 70, weight: 140 }
];

const compare = (a, b) => a.height - b.height;

// stable - sorted by height using stableSort()
// Items with the same height are still sorted by weight which means they preserved their relative order.
stableSort(input, compare);
/*
  [
    { height: 70, weight: 95 },
    { height: 70, weight: 125 },
    { height: 70, weight: 130 },
    { height: 70, weight: 140 },
    { height: 75, weight: 140 },
    { height: 80, weight: 110 },
    { height: 90, weight: 90 },
    { height: 100, weight: 80 },
    { height: 100, weight: 100 },
    { height: 100, weight: 120 },
    { height: 100, weight: 135 },
    { height: 110, weight: 115 }
  ]
*/

// unstable - sorted by height using Array.sort()
input.sort(compare);
/*
  [
    { height: 70, weight: 140},
    { height: 70, weight: 95 },
    { height: 70, weight: 125 },
    { height: 70, weight: 130 },
    { height: 75, weight: 140 },
    { height: 80, weight: 110 },
    { height: 90, weight: 90 },
    { height: 100, weight: 100 },
    { height: 100, weight: 80 },
    { height: 100, weight: 135 },
    { height: 100, weight: 120 },
    { height: 110, weight: 115 }
  ]
*/