text: 'use strict';
  
  /**
   * Is this value defined and not null?
   * @private
   */
  const defined = function (val) {
    return typeof val !== 'undefined' && val !== null;
  };
  
  /**
   * Is this value an object?
   * @private
   */
  const object = function (val) {
    return typeof val === 'object';
  };
  
  /**
   * Is this value a plain object?
   * @private
   */
  const plainObject = function (val) {
    return Object.prototype.toString.call(val) === '[object Object]';
  };
  
  /**
   * Is this value a function?
   * @private
   */
  const fn = function (val) {
    return typeof val === 'function';
  };
  
  /**
   * Is this value a boolean?
   * @private
   */
  const bool = function (val) {
    return typeof val === 'boolean';
  };
  
  /**
   * Is this value a Buffer object?
   * @private
   */
  const buffer = function (val) {
    return val instanceof Buffer;
  };
  
  /**
   * Is this value a Uint8Array?
   * @private
   */
  const uint8Array = function (val) {
    return val instanceof Uint8Array;
  };
  
  /**
   * Is this value a non-empty string?
   * @private
   */
  const string = function (val) {
    return typeof val === 'string' && val.length > 0;
  };
  
  /**
   * Is this value a real number?
   * @private
   */
  const number = function (val) {
    return typeof val === 'number' && !Number.isNaN(val);
  };
  
  /**
   * Is this value an integer?
   * @private
   */
  const integer = function (val) {
    return Number.isInteger(val);
  };
  
  /**
   * Is this value within an inclusive given range?
   * @private
   */
  const inRange = function (val, min, max) {
    return val >= min && val <= max;
  };
  
  /**
   * Is this value within the elements of an array?
   * @private
   */
  const inArray = function (val, list) {
    return list.includes(val);
  };
  
  /**
   * Create an Error with a message relating to an invalid parameter.
   *
   * @param {string} name - parameter name.
   * @param {string} expected - description of the type/value/range expected.
   * @param {*} actual - the value received.
   * @returns {Error} Containing the formatted message.
   * @private
   */
  const invalidParameterError = function (name, expected, actual) {
    return new Error(
      `Expected ${expected} for ${name} but received ${actual} of type ${typeof actual}`
    );
  };
  
  module.exports = {
    defined: defined,
    object: object,
    plainObject: plainObject,
    fn: fn,
    bool: bool,
    buffer: buffer,
    uint8Array: uint8Array,
    string: string,
    number: number,
    integer: integer,
    inRange: inRange,
    inArray: inArray,
    invalidParameterError: invalidParameterError
  };
  