0xD34F

const today = new Date().getDate();
const select = document.querySelector('select');
const toggle = option => option.disabled = option.value < today;

select.querySelectorAll('option').forEach(toggle);

// или

Array.prototype.forEach.call(select, toggle);

// или

for (const n of select.options) {
  toggle(n);
}

// или

for (let i = 0; i < select.children.length; i++) {
  toggle(select.children[i]);
}

// или

(function next(i, n = select.item(i)) {
  if (n) {
    toggle(n);
    next(-~i);
  }
})(0);

// или

const next = n => n && (next(n.nextElementSibling), toggle(n));
next(select.firstElementChild);

const duplicateContent = el =>

  // можно добавить копии вложенных узлов
  el.append(...el.cloneNode(true).childNodes);

  // или добавить копию разметки
  // el.insertAdjacentHTML('beforeend', el.innerHTML);

  // или перезаписать разметку в удвоенном виде
  // el.innerHTML += el.innerHTML;
  // el.innerHTML = el.innerHTML.repeat(2);
  // el.innerHTML = Array(3).join(el.innerHTML);
  // el.innerHTML = String.prototype.concat.apply('', Array(2).fill(el.innerHTML));
  // el.innerHTML = el.innerHTML.replace(/.+/, '$&$&');
  // el.innerHTML = /(.+)/.exec(el.innerHTML).join``;

document.querySelectorAll('.btn').forEach(duplicateContent);

// или

for (const n of document.getElementsByClassName('btn')) {
  duplicateContent(n);
}

const getDuplicatesIndex = (arr, key = n => n) =>
  Object.fromEntries(Object
    .entries(arr.reduce((acc, n, i) => ((acc[key(n)] ??= []).push(i), acc), {}))
    .filter(n => n[1].length > 1)
  );

const getDuplicatesIndex = (arr, key = n => n) =>
  new Map(Array
    .from(arr.reduce((acc, n, i) => {
      const k = key(n);
      acc.set(k, acc.get(k) ?? []).get(k).push(i);
      return acc;
    }, new Map))
    .filter(n => ~-n[1].length)
  );

function wrapper(fn) {
  function wrapped(...args) {
    return fn.apply(this, args);
  }

  wrapped.reset = () => console.log('hello, world!!');

  return wrapped;
}

const longestStr = arr.reduce((max, n) => max.length > n.length ? max : n, '');
// или
const longestStr = arr.sort((a, b) => b.length - a.length)[0];
// или
const longestStr = arr.reduce((acc, n) => (acc[n.length] = n, acc), []).pop();

function max(data, key = n => n) {
  const getVal = key instanceof Function ? key : n => n[key];
  let result = null;

  for (const n of data) {
    const val = getVal(n);
    if (!result || result[1] < val) {
      result = [ n, val ];
    }
  }

  return result?.[0];
}

const longestStr = max(arr, 'length');

document.querySelector('#check').addEventListener('change', e => {
  document.querySelector('#password-input').type = e.target.checked
    ? 'text'
    : 'password';
});

const getRepetition = (arr, repeated) => Array
  .from(arr.reduce((acc, n) => acc.set(n, -~acc.get(n)), new Map))
  .reduce((acc, n) => (n[1] === repeated && acc.push(n[0]), acc), []);

function getRepetition(arr, repeated) {
  const result = [];
  const count = {};

  for (const n of arr) {
    if (!count.hasOwnProperty(n)) {
      count[n] = 0;
    }

    count[n]++;
  }

  for (const n in count) {
    if (count[n] === repeated) {
      result.push(+n);
    }
  }

  return result;
}

$(document).click('.e-1', function(event) {
console.log(event.target)
});

$(document).on('click', '.e-1', function(e) {
  console.log(e.currentTarget);
});

const Format = (function createFormat(tags) {
  return new Proxy(
    (...content) => tags.reduceRight((acc, n) => `<${n}>${acc}</${n}>`, content.join('')),
    { get: (target, key) => createFormat([ ...tags, key ]) }
  );
})([]);

const random = arr => arr[Math.random() * arr.length | 0];

const item = random(random(Object.values(state.themes)));

const item = random(Object.values(state.themes).flat());

function weightedRandom(arr, key = () => 1) {
  const val = key instanceof Function ? key : n => n[key];
  const max = arr.reduce((acc, n) => acc + val(n), 0);

  return () => {
    let rand = Math.random() * max;
    return arr.find(n => (rand -= val(n)) < 0);
  };
}

const randomArr = weightedRandom(Object.values(state.themes), 'length');

// ...

const item = random(randomArr());

const elements = Array.prototype.filter.call(
  document.querySelectorAll('.green'),
  (n, i, a) => n.nextElementSibling !== a[i + 1]
);

str.split('/').pop()
// или
str.match(/[^\/]+$/)[0]
// или
str.replace(/.*\//, '')
// или
str.slice(str.lastIndexOf('/') + 1)
// или
Array.from(str).reduce((acc, n) => n === '/' ? '' : acc + n, '')
// или
[...str].filter((n, i, a) => !a.includes('/', i)).join('')

const itemSelector = '.header';
const buttonSelector = `${itemSelector} .delete_block`;

document.querySelectorAll(buttonSelector).forEach(function(n) {
  n.addEventListener('click', this);
}, ({ currentTarget: t }) => {
  while (!(t = t.parentNode).matches(itemSelector)) ;
  t.replaceWith();
});

// или

document.addEventListener('click', e => e
  .target
  .closest(buttonSelector)
  ?.closest(itemSelector)
  .remove()
);

function merge(key, ...arrs) {
  const getKey = key instanceof Function ? key : n => n[key];
  const result = new Map;

  arrs.forEach(arr => arr.forEach(n => {
    const k = getKey(n);
    result.set(k, Object.assign(result.get(k) ?? {}, n));
  }));

  return [...result.values()];
}

const getTruthyKeys = obj =>
  Object
    .entries(obj)
    .filter(n => n[1])
    .map(n => n[0]);

for([key, value] of Object.entries(item)) {

const obj = new Proxy({
  a: 0,
  b: 1,
  c: 2,
}, {
  get: () => Math.round(Math.random()),
});

console.log(Array.from({ length: 10 }, () => getTruthyKeys(obj)));

const replaceValues = (val, test, replacer) =>
  val instanceof Array
    ? val.map(n => replaceValues(n, test, replacer))
    : val instanceof Object
      ? Object.fromEntries(Object
          .entries(val)
          .map(([ k, v ]) => [
            k,
            test(k, v)
              ? replacer(v)
              : replaceValues(v, test, replacer)
          ])
        )
      : val;

const newData = replaceValues(
  data,
  k => k.includes('Date'),
  v => v.replace(/(\d+)-(\d+)-/, '$2.$1.')
);

$('.content_toggle').click(function() {
  const $button = $(this);

  $(`#box-${this.id.split('-').pop()}`).slideToggle(300, function() {
    const isHidden = $(this).is(':hidden');
    $button
      .text(isHidden ? 'Показать текст' : 'Скрыть текст')
      .toggleClass('open', !isHidden);
  });

  return false;
});

$(this).closest('здесь селектор общего предка').find('.content_block')

$('.content_block').eq($('.content_toggle').index(this))