0xD34F

const items = document.querySelectorAll('.faq__item');

items.forEach(n => n.addEventListener('change', onChange));

function onChange(e) {
  items.forEach(n => n
    .querySelector('.faq__question--top')
    .classList
    .toggle('active', e.currentTarget === n)
  );
}

array2.filter(function(n) {
  return this.has(n.code);
}, new Set(array1.map(n => n.value)))

array1.map(function(n) {
  return this[n.value];
}, Object.fromEntries(array2.map(n => [ n.code, n ])))

/*
 * если вдруг в array1 могут быть объекты, value которых отсутствует среди code в array2,
 * то после map'а надо будет сделать фильтрацию, .filter(Boolean), например
 */

array2.filter(n => array1.some(m => m.value === n.code))

e.target.dataset.followerStyle

Object.values(arr.reduce((acc, n) => (
  acc[n.group] = acc[n.group]?.score > n.score ? acc[n.group] : n,
  acc
), {}))

// значения withSeat вместо раскладывания в отдельные переменные собираете в объект
const withSeat = {
  adult: 1,
  teenager: 1,
  babe: 0,
};


const passengers = cabins.flatMap(n => Object
  .entries(n)
  .flatMap(([ k, v ]) => Array.from(
    { length: v },
    () => ({ withSeat: withSeat[k] })
  ))
);

arr.reduce((acc, n, i, a) => (
  a[i - 1]?.name[0] !== n.name[0] && acc.push({ name: n.name[0] }),
  acc.push(n),
  acc
), [])

const obj = Object.fromEntries(Object
  .entries(arr.reduce((acc, n) => (
    Object.entries(n).forEach(([ k, v ]) => (acc[k] ??= new Set).add(v)),
    acc
  ), {}))
  .map(n => [ n[0], n[1].size === 1 ? [...n[1]][0] : null ])
);

formatDate(date) {
  const today = new Date();
  return [ 'getFullYear', 'getMonth', 'getDate' ].every(n => date[n]() === today[n]())
    ? 'Today'
    : flatpickr.formatDate(date, 'j M');
},

document.addEventListener('change', ({ target: t }) => {
  const fieldset = t.closest('fieldset');
  if (fieldset) {
    const [ main, ...items ] = fieldset.querySelectorAll('input[type="checkbox"]');
    if (main === t) {
      items.forEach(n => n.checked = t.checked);
    } else {
      main.checked = items.every(n => n.checked);
    }

    main.closest('legend').classList.toggle('active', main.checked);
  }
});

document.querySelector('.result').append(...Array
  .from(document.querySelector('.data').cloneNode(true).children, n => [ n, +n.innerText ])
  .sort((a, b) => a[1] - b[1])
  .map(n => n[0])
);

document.querySelectorAll('.tariff__counter').forEach(n => {
  const max = n.dataset.seats;
  n.querySelectorAll('.tariff__counter-max').forEach(m => m.innerText = max);
  n.querySelectorAll('.amount').forEach(m => m.max = max);
});

document.addEventListener('click', ({ target: t }) => {
  const change = ({
    '-':  1,
    '+': -1,
  })[t.innerText];

  if (change) {
    const input = t.closest('.tariff__counter-block').querySelector('.amount');
    input.value -= change;
    input.dispatchEvent(new Event('input', { bubbles: true }));
  }
});

document.addEventListener('input', ({ target: t }) => {
  if (t.classList.contains('amount')) {
    t.value = Math.max(t.min, Math.min(t.max, t.value));
  }
});

function getLeaves(tree) {
  const result = [];
  const arr = [...tree];

  for (const n of arr) {
    if (Array.isArray(n.children) && n.children.length) {
      arr.push(...n.children);
    } else {
      result.push(n);
    }
  }

  return result;
}

function getLeaves(tree) {
  const result = [];
  const stack = [];

  for (let arr = tree, i = 0; i < arr.length || stack.length; i++) {
    if (i === arr.length) {
      [ arr, i ] = stack.pop();
    } else if (Array.isArray(arr[i].children) && arr[i].children.length) {
      stack.push([ arr, i ]);
      arr = arr[i].children;
      i = -1;
    } else {
      result.push(arr[i]);
    }
  }

  return result;
}

const groupSum = (arr, keyProp, ...sumProps) =>
  Object.values(arr.reduce((acc, n) => (
    acc[n[keyProp]] ??= { ...n, ...Object.fromEntries(sumProps.map(m => [ m, 0 ])) },
    sumProps.forEach(m => acc[n[keyProp]][m] += n[m]),
    acc
  ), {}));


const result = groupSum(arr, 'product', 'price', 'selling_price', 'quantity');

document.querySelectorAll('.elem button').forEach(n => {
  n.addEventListener('click', onPlusMinusClick);
});

document.querySelectorAll('.btn-all').forEach(n => {
  n.addEventListener('click', onGetDataClick);
});


function onPlusMinusClick({ target }) {
  const input = target.closest('.elem').querySelector('input');
  const change = target.innerText === '+' ? -1 : 1;
  input.value = Math.max(input.min, Math.min(input.max, input.value - change));
}

function onGetDataClick({ target }) {
  const data = Array
    .from(target.closest('.child-block').querySelectorAll('.elem'))
    .flatMap(n => Array(+n.querySelector('input').value).fill(+n.dataset.id));

  console.log(data);
}

const obj = {
  a: {
    b: {
      c: {
        xxx: 666,
      },
    },
  },
};

const proxy = new Proxy(obj, {
  get: (target, key) => key.split('.').reduce((p, c) => p?.[c], target),
});

console.log(proxy['a.b.c.xxx']); // 666