Filter an array of people by age and group them by gender.

filterAndGroup.js

function filterAndGroup(data, filterFunc, groupByKey) {
  return data.reduce((acc, item) => {
    if (filterFunc(item)) {
      if (item[groupByKey] in acc) {
        acc[item[groupByKey]].push(item);
      } else {
        acc[item[groupByKey]] = [item];
      }
    }

    return acc;
  }, {});
}

function filterBy30to40AndGroupByGender(data) {
  return filterAndGroup(
    data,
    (_) => _.age > 30 && _.age < 40,
    'gender'
  )
}