A function for partitioning an array based on the result of a filter function, rather than discarding the failed filter results. Sane people can also just run two `Array.filter()` methods.

partitionArray.js

/**
 * Divides one array into two based on a filter function, returning the results
 * of the filter as a [pass, fail] tuple.
 *
 * @param array          The array to partition into separate arrays based on the provided filter.
 * @param filterFunction The filter function to use to determine how each array item should be partitioned.
 *
 * @returns A tuple of partitioned array items, functionally equivalent to `[pass, fail]` in relation to the provided
 *          filter function.
 */
function partitionArray(array, filterFunction) {
  return array.reduce(
    (groups, item) => {
      // Determine if this item should be placed in the pass (0) or fail (1)
      // group based on the results of the filter function.
      const groupIndex = filterFunction(item) ? 0 : 1;

      // Place the item in the appropriate group.
      groups[groupIndex].push(item);

      return groups;
    },
    [[], []]
  );
}

partitionArray.ts

/**
 * Divides one array into two based on a filter function, returning the results
 * of the filter as a [pass, fail] tuple.
 *
 * @param array          The array to partition into separate arrays based on the provided filter.
 * @param filterFunction The filter function to use to determine how each array item should be partitioned.
 *
 * @returns A tuple of partitioned array items, functionally equivalent to `[pass, fail]` in relation to the provided
 *          filter function.
 */
function partitionArray<TItem>(
  array: TItem[],
  filterFunction: (item: TItem) => boolean
): [TItem[], TItem[]] {
  return array.reduce(
    (groups, item) => {
      // Determine if this item should be placed in the pass (0) or fail (1)
      // group based on the results of the filter function.
      const groupIndex = filterFunction(item) ? 0 : 1;

      // Place the item in the appropriate group.
      groups[groupIndex].push(item);

      return groups;
    },
    [[], []] as [TItem[], TItem[]]
  );
}