indexBy

Problem

You have an array and need O(1) lookup by a unique key — for example building an id-to-object map from an API response.

Solution

Use indexBy(array, selector) to produce a Record<string, T> keyed by the selector result.

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

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
];

const byId = indexBy(users, (u) => u.id);
byId[1]; // { id: 1, name: 'Alice' }
byId[2]; // { id: 2, name: 'Bob' }

Pitfalls

• If multiple items share the same key, the last one wins.
• Keys are always coerced to strings — numeric ids become string keys in the record.

Related

• groupBy
• countBy