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not ​

Problem ​

You have a predicate and need its inverse — for example filtering out items that match a condition, or composing negated guards.

Solution ​

Use not(predicate) to return a new function that inverts the boolean result.

ts
import { not } from '@vielzeug/arsenal';

const isActive = (user: { active: boolean }) => user.active;
const isInactive = not(isActive);

const inactiveUsers = users.filter(isInactive);

// Negate inline with Array methods
const validItems = formItems.filter(not((item) => item.errors.length > 0));

Pitfalls ​

  • not only inverts truthiness — it does not produce a TypeScript type narrowing predicate. Add an explicit (x): x is T overload if you need narrowed types on the result.
  • Prefer noneOf(pred) over not(pred) when you need to combine multiple predicates at once.