code: function checkIfOnlyOneRequired (defaultParams) {
    var keys = Object.keys(defaultParams);
    var i, key;
    var onlyOne = '';
    
    for (i = 0; i < keys.length; i++){
      key = keys[i];
      if (defaultParams[key] === '') {
        if (onlyOne !== '') return false;
        onlyOne = key;
      }
    }
    
    return onlyOne;
  }
docs: Check if default parameters has only one required param
  
  Returns false if there are more than one, or a param name if there is only one