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Performs stable sorting of an array, preserving the initial indexes of items when their values are the same. Does not mutate the original array, but returns a new array instead.

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.

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