code: function iter(obj, each) {
    if (!obj || typeof obj !== 'object') { return ''; }
  
    let output = '';
    const keys = Object.keys(obj);
    const length = keys.length;
  
    for (let i = 0; i < length; i += 1) {
      const key = keys[i];
      output += each(key, i, length, obj[key]);
    }
  
    return output;
  }
docs: Iterate over an object or array
  @param {string[]} obj - Iteratee object / array
  @param {function} each - Callback to execute on each item
  @return {string}