code: function makeSubfieldPairs(subfields1, subfields2, fn_comparator) {
        
        var pairs = [];
  
        if (subfields1.length === subfields2.length) {
  
          subfields2.forEach(function(subfield2) {          
            return subfields1.some(function(subfield1) {
              if (fn_comparator(subfield1, subfield2)) {
                
                pairs.push([subfield1, subfield2]);
                return true;
                
              }
            });
          });
  
        } else {
          throw new Error('Number of subfields are not equal');
        }
  
        return pairs;
        
      }
docs: Try to make a list of equal pairs from both subfield arrays using a comparator function