0xD34F

if (!$('.work-album-item:hidden').length) {
  $(this).hide();
}

// или

$(this).toggle(!!$('.work-album-item:hidden').length);

Promise
  .all([...img].map(n => new Promise(r => n.complete ? r() : n.onload = r)))
  .then(() => img.forEach(n => n.style.position = 'absolute'));

const intervals = [ 1000, 2000, 500, 3000 ];
let interval = -1;

function newsRotator() {
  const $news = $('.news');
  const $hidden = $news.not(':visible');
  const $next = $hidden.eq(Math.random() * $hidden.length | 0);

  $news.hide();
  $next.show();

  interval = (interval + 1) % intervals.length;
  setTimeout(newsRotator, intervals[interval]);
}

newsRotator();

for(i = 0

i<= n;

newArr[i] = a[i];

function arrayNtimes(a, n) {
  const newArr = [];

  for (let i = 0; i < n * a.length; i++) {
    newArr[i] = a[i % a.length];
  }

  return newArr;
}

const arrayNtimes = (a, n) => Array(n).fill(a).flat();

const titleCase = str => str
  .toLowerCase()
  .split(' ')
  .map(n => n[0].toUpperCase() + n.slice(1))
  .join(' ');

const titleCase = str => str
  .toLowerCase()
  .replace(/(?<=^| )./g, m => m.toUpperCase());

const titleCase = str => Array
  .from(str, (n, i) => n[i && str[i - 1] !== ' ' ? 'toLowerCase' : 'toUpperCase']())
  .join``;

const container = document;
const selector = 'input[type="checkbox"]';
const sumElements = elements =>
  Array.prototype.reduce.call(elements, (acc, n) => acc + +n.value, 0);
const onSumChange = sum => console.log(sum);

container.addEventListener('change', function({ target: t }) {
  if (t.matches(selector)) {
    onSumChange(sumElements(this.querySelectorAll(`${selector}:checked`)));
  }
});

container.querySelector(selector).dispatchEvent(new Event('change', { bubbles: true }));

const cb = container.querySelectorAll(selector);
let sum = sumElements(Array.prototype.filter.call(cb, n => n.checked));
const onChange = e => onSumChange(sum += e.target.value * [ -1, 1 ][+e.target.checked]);
cb.forEach(n => n.addEventListener('change', onChange));
onSumChange(sum);

el.addEventListener('input', function() {
  this.value = this.value.replace(/[^а-яё\s]/ig, '');
});

str.slice(1).split('&').reduce((acc, n) => (n = n.split('='), acc[n[0]] = n[1], acc), {})

// или

[...str.matchAll(/(\w+)=([^&]*)/g)].reduce((acc, [ , k, v ]) => ({ ...acc, [k]: v }), {})

// или

Object.fromEntries(new URLSearchParams(str))

const STAGES = {
  2: 'itemGv',
  5: 'itemTm',
};

for (const [ key, val ] of Object.entries(items)) {
  const s = STAGES[val.stage];
  if (s) {
    this.client.srem('items', key);
    this.emit(s, val);
  }
}

const XXX = length => Array
  .from({ length }, (n, i) => Array
    .from({ length: i + 1 }, (n, i) => i + 1)
    .join(','))
  .join('\n');

console.log(XXX(5));

const XXX = length => Array
  .from({ length }, function() {
    this.push(-~this.length);
    return this.join`,`;
  }, [])
  .join`\n`;

const XXX = length => Array
  .from({ length }, (n, i) => Array
    .from(Array(++i).keys())
    .reduce((acc, n) => acc + (acc && ',') + ++n, ''))
  .reduce((acc, n) => acc + (acc && '\n') + n, '');

const SKILLS = {
  sport: [ 'running', 'squats', 'snowboarding' ],
  music: [ 'guitar', 'drums' ],
};

const filterBySkill = (people, filter) =>
  people.filter(({ skills }) =>
    skills.every(n => filter.some(m => SKILLS[m].includes(n))) &&
    filter.every(n => skills.some(m => SKILLS[n].includes(m)))
  );

function setTimer(el, template) {
  return setInterval(el => {
    const d = new Date();
    const timeData = Object.fromEntries([
      [ 'h', 'getHours' ],
      [ 'm', 'getMinutes' ],
      [ 's', 'getSeconds' ],
    ].map(n => [ n[0], `${d[n[1]]()}`.padStart(2, 0) ]));

    el.innerHTML = template.replace(
      /\$(\w)/g,
      (m, g1) => timeData.hasOwnProperty(g1) ? timeData[g1] : m
    );
  }, 1000, typeof el === 'string' ? document.querySelector(el) : el);
}


const intervalId = setTimer('#time', '<strong>Сейчас</strong> $h:$m:$s');

function sortChildren(el, key) {
  el.append(...Array
    .from(el.children, n => [ n, key(n) ])
    .sort((a, b) => a[1] - b[1])
    .map(n => n[0])
  );
}


sortChildren(document.querySelector('.menu'), n => +n.dataset.num);

• Замените document.getElementsByTagName("p") на document.querySelectorAll('p'), так вы получите статическую коллекцию элементов.
  
• Перебирайте содержимое allP от конца к началу:
  
  

  for (let i = allP.length; i--;) {
    body.removeChild(allP[i]);
  }

  
  
• Удаляйте элемент с нулевым индексом, пока есть что удалять:
  
  

  while (allP.length) {
    allP[0].remove();
  }

  
  
  

как улучшить мой код

function fearNotLetter(str) {
  const missing = Array
    .from(str, n => n.charCodeAt(0))
    .find((n, i, a) => n !== a[0] + i);

  return missing && String.fromCharCode(missing - 1);
}

Array.prototype.unique = function() {
  this.splice(0, this.length, ...new Set(this));
  return this;
};

Array.prototype.unique = function(key = n => n) {
  const getKey = key instanceof Function ? key : n => n[key];
  let numDeleted = 0;

  this.forEach(function(n, i, a) {
    a[i - numDeleted] = n;
    const k = getKey(n);
    numDeleted += this.has(k) || !this.add(k);
  }, new Set);

  this.length -= numDeleted;

  return this;
};


[ 1, 1, 1, 2 ].unique() // [1, 2]
[ { id: 3 }, { id: 1 }, { id: 1 }, { id: 3 } ].unique('id') // [{id: 3}, {id: 1}]
[ 'a', 'b', 'c', 'd', 'ab', 'bc' ].unique(n => n.length) // ['a', 'ab']