froebel/partition.mjs

TypeScript utility library

/**
 * Takes a `list` and returns a pair of lists containing: the elements that
 * match the `predicate` and those that don't, respectively.
 *
 * Think of it as `filter`, but the elements that don't pass the filter aren't
 * discarded but returned in a separate list instead.
 *
 * @example
 * ```
 * const [strings, numbers] = partition(
 *   ['a', 'b', 1, 'c', 2, 3],
 *   (el): el is string => typeof el === 'string'
 * )
 * // strings: ["a", "b", "c"]
 * // numbers: [1, 2, 3]
 * ```
 */
const partition = (list, predicate) => list.reduce(([t, f], c) => predicate(c) ? [[...t, c], f] : [t, [...f, c]], [[], []]);

export default partition;