partition

Problem

You need to split an array into two groups — items that pass a predicate and items that fail — in a single pass.

Solution

Use partition(array, predicate) to receive a [pass[], fail[]] tuple.

import { partition } from '@vielzeug/arsenal';

const [evens, odds] = partition([1, 2, 3, 4, 5], (n) => n % 2 === 0);
// evens: [2, 4]
// odds:  [1, 3, 5]

const [admins, users] = partition([{ role: 'admin' }, { role: 'user' }, { role: 'admin' }], (u) => u.role === 'admin');

Pitfalls

• Both groups are always returned, even if empty — never undefined.
• Order within each group follows the original array.

Related

• filterMap
• groupBy
• compact