test

const extractValues = (
  fields: string[],
  data: DataItemType[]
): Record<string, string[]> => {
  if (!fields.length) {
    return {};
  }
  const fieldsWithValues: Record<string, string[]> = {};

  for (let item of data) {
    for (let field of fields) {
      fieldsWithValues[field] = fieldsWithValues[field] || [];

      const val = getFieldsByPath(item, field).join(', ');

      fieldsWithValues[field].push(val);
    }
  }

  Object.entries(fieldsWithValues).forEach(([field, values]) => {
    fieldsWithValues[field] = sortArrayOfStringsByDefaultOrder(
      values,
      field,
      null
    );
  });
  return fieldsWithValues;
};

const mockItems = [
        { a: 'a1', b: 'b1', c: 'c1' },
        { a: 'a2', b: 'b2', c: 'c2' },
        { a: 'a3', b: 'b3', c: 'c3' },
      ];

      const extracted = extractValues(['a', 'b'], mockItems);

const extractValues = (
  fields: string[],
  data: DataItemType[]
): Record<string, string[]> => {
  if (!fields.length) {
    return {};
  }
  const fieldsWithValues = data.reduce(
    (acc, item) =>
      fields.reduce((acc, field) => {
        if (!acc.has(field)) {
          acc.set(field, new Set());
        }
        acc.get(field)!.add(getFieldsByPath(item, field).join(', '));
        return acc;
      }, acc),
    new Map()
  );
  const entries = [...fieldsWithValues.entries()].map(([field, values]) => [
    field,
    sortArrayOfStringsByDefaultOrder([...values], field, null),
  ]);
  return Object.fromEntries(entries);
};


const mockItems = [
        { a: 'a1', b: 'b1', c: 'c1' },
        { a: 'a2', b: 'b2', c: 'c2' },
        { a: 'a3', b: 'b3', c: 'c3' },
      ];

      const extracted = extractValues(['a', 'b'], mockItems);