Partition an array in one pass

Writing items.filter(isDone) and items.filter(x => !isDone(x)) walks the array twice and calls the predicate twice per item. When the predicate is expensive, or when you want the two halves to stay in sync, one pass is better.

Implement partition(items, predicate). Call predicate(item, index) for each item, where index is its position in the input array. Return an array of exactly two arrays: the first holds every item for which the predicate returned a truthy value, the second holds the rest. Both keep the items in their original relative order. An empty input returns [[], []]. Never modify items.

partition([1, 2, 3, 4], (n) => n % 2 === 0);
// [[2, 4], [1, 3]]

partition(["a", "b", "c", "d"], (_, index) => index % 2 === 0);
// [["a", "c"], ["b", "d"]] — the predicate gets the index too

partition([0, 1, "", "x"], (value) => value);
// [[1, "x"], [0, ""]] — truthiness, not strict equality