import {
  isNil,
  isNotNil,
  isString,
  nullable,
  nonNull,
  nullish,
  optional,
  required,
} from 'is-kit';

isNil(null); // true
isNil(undefined); // true
isNil(0); // false
// isNil checks a value directly for null | undefined.

const values: Array<string | null | undefined> = ['value', null, undefined];
const presentValues = values.filter(isNotNil); // string[]
// isNotNil excludes null | undefined and preserves generic narrowing.

const maybeString = nullable(isString);
maybeString(null); // true

const notNull = nonNull(isString);
notNull('x'); // true

const maybe = nullish(isString);
maybe(undefined); // true
// nullish(isString) widens isString to also accept null or undefined.

const maybeUndef = optional(isString);
maybeUndef(undefined); // true

const needValue = required(optional(isString));
needValue('ok'); // true

// isNil checks the value itself.
// nullish(...) widens another guard to accept null or undefined.
// optional(...) is value-level.
// For struct key-level optional properties, use optionalKey(...).