countBy

Problem

You have a collection and need to count occurrences grouped by a computed key — for example tallying users by role.

Solution

Use countBy(array, selector) to produce a Record<string, number> of key → count pairs.

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

const users = [{ role: 'admin' }, { role: 'user' }, { role: 'user' }, { role: 'admin' }, { role: 'guest' }];

countBy(users, (u) => u.role);
// { admin: 2, user: 2, guest: 1 }

Pitfalls

• Keys are always strings (coerced via selector return value). A selector returning a number produces string keys.

Related

• groupBy
• indexBy