import { OperatorFunction } from 'rxjs';
/**
 * Creates an operator function that filters out `null` and `undefined` values from an Observable.
 *
 * This function is a higher-order function that returns an `OperatorFunction`. The returned operator function
 * takes an Observable that emits values of type `T` and returns an Observable that emits only values of type `T`
 * that are neither `null` nor `undefined`, effectively narrowing the type from `T` to `NonNullable<T>`.
 *
 * @template T - The type of items emitted by the source Observable.
 * @returns An `OperatorFunction<T, NonNullable<T>>` that filters out `null` and `undefined` values from the source Observable.
 *
 * ### Example
 * ```typescript
 * import { of } from 'rxjs';
 * import { isDefined } from './isDefined';
 *
 * const source$ = of(1, 2, null, 3, undefined, 4);
 * const filtered$ = source$.pipe(isDefined());
 * filtered$.subscribe(console.log); // Output: 1, 2, 3, 4
 * ```
 */
export declare function isDefined<T>(): OperatorFunction<T, NonNullable<T>>;