filter-array-objects

'use strict';

const data = [
    {name: 'Adam', age: 30, gender: 'Male'},
    {name: 'Nancy', age: 40, gender: 'Female'},
    {name: 'Fred', age: 35, gender: 'Male'},
    {name: 'Nina', age: 21, gender: 'Female'},
    {name: 'Chloe', age: 37, gender: 'Female'},
    {name: 'Neo', age: 32, gender: 'Male'},
    {name: 'Max', age: 39, gender: 'Male'},
];


const formattedOutput = filterData(data);

console.log(formattedOutput);

function filterData (persons) {
    if (!Array.isArray(persons)) {
        return false;
    }
    const filteredPerson = persons.filter((person) => {
        return (person.age > 30 && person.age < 40);
    });

    return groupBy(filteredPerson, person => person.gender);
}

function groupBy (array, func) {
    const result = array.reduce((r, v, i, a, k = func(v)) => ((r[k] || (r[k] = [])).push(v), r), {});
    return result;
}