text: 'use strict';
  
  exports.isString = obj => typeof obj === 'string';
  
  exports.isFunction = fn => typeof fn === 'function';
  
  exports.isError = err => err instanceof Error;
  
  exports.flatten = function flatten(arr, ret) {
    ret = ret || [];
    for (let elem of arr) {
      if (Array.isArray(elem)) {
        flatten(elem, ret);
      } else {
        ret.push(elem);
      }
    }
  
    return ret;
  };
  