code: function createRandomStr(length) {
    const bag = 'abcdefghijklmnopqrstuvwxyz0123456789';
    let generated = '';
    for (let i = 0; i < length; i++) {
      generated += bag[Math.floor(Math.random() * bag.length)];
    }
    return generated;
  }
docs: @private
  
  create random strings
  @see http://qiita.com/ryounagaoka/items/4736c225bdd86a74d59c
  
  @param {number} length
  @return {string} random strings