/**
 * isUndefined
 * Checks if a value is undefined or not.
 *
 * @name isUndefined
 * @function
 * @param {Anything} input The input value.
 * @returns {Boolean} `true`, if the input is `undefined`, `false` otherwise.
 */
module.exports = function isUndefined(input) {
    return typeof input === 'undefined';
};

"use strict";

/**
 * isUndefined
 * Checks if a value is undefined or not.
 *
 * @name isUndefined
 * @function
 * @param {Anything} input The input value.
 * @returns {Boolean} `true`, if the input is `undefined`, `false` otherwise.
 */
let u = void 0;
module.exports = input => input === u;

Use void 0 to get the undefined value.