/**
 * Verifies that the input is not `undefined`, `null`, or `NaN`.
 * @param input The value to check.
 * @returns `true` if the input is not `undefined`, `null`, or `NaN`, `false` otherwise.
 */
export declare const isDefined: <T>(input: T) => input is Exclude<T, undefined | null>;
/**
 * Verifies that the input is `undefined`.
 * @param input The value to check.
 * @returns `true` if the input is `undefined`, `false` otherwise.
 */
export declare const isUndefined: (input: unknown) => input is undefined;
/**
 * Verifies that the input is `null`.
 * @param input The value to check.
 * @returns `true` if the input is `null`, `false` otherwise.
 */
export declare const isNull: (input: unknown) => input is null;
/**
 * Verifies that the input is `undefined` or `null`.
 * @param input The value to check.
 * @returns `true` if the input is `undefined` or `null`, `false` otherwise.
 */
export declare const isNil: (input: unknown) => input is undefined | null;