/**
 * Helper function to determine if input is undefined.
 *
 * @param input - the input being tested
 * @returns boolean
 *
 * @example
 * isUndefined(undefined) // Returns: true
 * isUndefined(null)      // Returns: false
 * isUndefined('foo')     // Returns: false
 */
export declare const isUndefined: (input: any) => input is undefined;