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filterMap ​

Problem ​

You need to transform an array and drop some items in a single pass — without a separate filter then map chain.

Solution ​

Use filterMap(array, fn) to map items and skip any where the callback returns undefined.

ts
import { filterMap } from '@vielzeug/arsenal';

const values = [1, null, 2, null, 3];

// Return undefined to drop; return a value to keep
filterMap(values, (n) => (n == null ? undefined : n * 2));
// [2, 4, 6]

Transform and narrow type ​

ts
import { filterMap } from '@vielzeug/arsenal';

type Raw = { value: string | null };
const rows: Raw[] = [{ value: 'a' }, { value: null }, { value: 'b' }];

const strings: string[] = filterMap(rows, (r) => r.value ?? undefined);
// ['a', 'b']

Pitfalls ​

  • Return undefined to drop an item. Returning null keeps it.
  • Not lazy — processes the whole array in one pass.