0xD34F

const isAllUnique = (...args) => new Set(args).size === args.length;

console.log(isAllUnique(69, 187, 666)); // true
console.log(isAllUnique(2, 2, 3, 4)); // false
console.log(isAllUnique(...'ABCDA')) // false

const elems = [...document.querySelectorAll('.class1')];
const index = elems.findIndex(n => n.classList.contains('class2'));

const elems = document.querySelectorAll('.class1');
let index = elems.length;
while (--index >= 0 && !elems[index].matches('.class2')) ;

const index = Array.prototype.indexOf.call(
  document.querySelectorAll('.class1'),
  document.querySelector('.class2')
);

<div class="country">
  <input class="country-code">
  <img class="country-flag">
</div>

function updateFlag(el) {
  const country = countries.find(n => !el.value.indexOf(n.code));
  el.closest('.country').querySelector('.country-flag').src = country ? country.flag : '';
}

document.addEventListener('input', function(e) {
  if (e.target.classList.contains('country-code')) {
    updateFlag(e.target);
  }
});

document.querySelectorAll('.country-code').forEach(updateFlag);

const countries = [
  { code: ..., flag: ... },
  { code: ..., flag: ... },
  ...
];

const country = countries.find(n => !inputCode.value.indexOf(n.code));
inputFlag.src = country ? country.flag : '';

<input type="number" min="0" max="100">
<table>
  <tbody></tbody>
</table>

const input = document.querySelector('input');
const table = document.querySelector('table');

input.addEventListener('input', ({ target: t }) => {
  const count = t.value = Math.max(0, t.value | 0);
  const [ tbody ] = table.tBodies;
  const { rows } = tbody;

  for (; rows.length > count; tbody.deleteRow(-1)) ;

  while (rows.length < count) {
    const row = tbody.insertRow();
    row.insertCell().textContent = '#' + rows.length;
    row.insertCell().textContent = Math.random() * 1e3 | 0;
  }
});

// или

input.oninput = function() {
  const count = this.value = Math.max(0, +this.value || 0);
  const tbody = table.querySelector('tbody');
  const rows = tbody.children;

  Array.prototype.slice.call(rows, Math.min(rows.length, count)).forEach(n => n.remove());

  tbody.insertAdjacentHTML('beforeend', Array
    .from({ length: Math.max(0, count - rows.length) }, (n, i) => `
      <tr>
        <td>#${rows.length + i + 1}</td>
        <td>${Math.floor(Math.random() * 1000)}</td>
      </tr>`)
    .join('')
  );
};

const $el = $('.banner');
const baseWidth = $el.width();

$(window).scroll(function() {
  $el.width(baseWidth + $(this).scrollTop());
});

const el = document.querySelector('.banner');
const baseWidth = el.offsetWidth;

document.addEventListener('scroll', () => {
  el.style.width = `${baseWidth + document.documentElement.scrollTop}px`;
});

document.addEventListener('click', function(e) {
  if (!e.target.closest('.tileset-showitems')) {
    tilesetShowListRemove();
  }
});

- .active {
+ .active .box {

const containerSelector = '.tileset-showitems';
const buttonSelector = `${containerSelector} .tileset-showitems-trigger`;
const activeClass = 'active';
const closeAllExcept = container => document
  .querySelectorAll(`${containerSelector}.${activeClass}`)
  .forEach(n => n !== container && n.classList.remove(activeClass));

document.addEventListener('click', ({ target: t }) => {
  const button = t.closest(buttonSelector);
  const container = t.closest(containerSelector);

  if (button) {
    container.classList.toggle(activeClass);
  }

  closeAllExcept(container);
});

window.addEventListener('keydown', e => e.key === 'Escape' && closeAllExcept());

for (const n of el.options) {
  for (const m of elValue) {
    if (n.value === m) {
      n.selected = true;
      break;
    }
  }
}

Array.prototype.forEach.call(
  el,
  n => n.selected = elValue.includes(n.value)
);

el
  .querySelectorAll(elValue.map(n => `[value="${n}"]`))
  .forEach(n => n.selected = true);

а где в верстке у тебя эти точки указаны?

const points = [
  { max:      200, text:  'hello, world!!' },
  { max:      800, text:  'fuck the world' },
  { max: Infinity, text: 'fuck everything' },
];

window.addEventListener('scroll', function() {
  const
    scroll = document.documentElement.scrollTop,
    { text } = points.find(n => n.max >= scroll);

  document.querySelector('.fixed').textContent = text;
});

const newArr = arr.map(function({ ...n }) {
  return this.reduce((acc, k) => (acc[k] = acc[k].toLowerCase(), acc), n);
}, [ 'firstName', 'lastName' ]);

const keys = [ 'firstName', 'lastName' ];
arr.forEach(n => keys.forEach(k => n[k] = n[k].toLowerCase()));

const data = $('table tr')
  .get()
  .map(tr => $('input', tr).get().map(n => $(n).val()))
  .filter(row => row[row.length - 1])
  .map(row => `(${row.map(n => `'${n}'`).join(', ')})`);

const data = Array.prototype.reduce.call(
  document.querySelector('table').rows,
  (acc, { cells, lastElementChild: last }) => (
    last.lastElementChild.value && acc.push(`(${Array
      .from(cells, n => `'${n.lastElementChild.value}'`)
      .join(', ')
    })`),
    acc
  ),
  []
);

Как поправить код?

const discounts = [
  { percent: 0, sum: 0 },
  { percent: 2, sum: 4000 },
  { percent: 3, sum: 10000 },
  { percent: 4, sum: 20000 },
  { percent: 5, sum: 50000 },
];

const discount = discounts.reduce((p, c) => c.sum <= pricechange ? c : p);

const discountedPrice = pricechange * (100 - discount.percent) / 100;

const discounts = [
  { percent: 0, sum: 4000 },
  { percent: 2, sum: 10000 },
  { percent: 3, sum: 20000 },
  { percent: 4, sum: 50000 },
  { percent: 5, sum: Infinity },
];

const discount = discounts.find(n => n.sum > pricechange);

const buttons = document.querySelectorAll('.item1');
const targets = document.querySelectorAll('.item2');
const className = 'red';

const updateClass = action => ({ target: t }) => {
  const index = Array.prototype.indexOf.call(buttons, t);
  if (index !== -1) {
    targets[index].classList[action](className);
  }
};

const wrapper = document.querySelector('#wrap1');
wrapper.addEventListener('click', updateClass('add'));
wrapper.addEventListener('dblclick', updateClass('remove'));

let state = true;

document.querySelector('button').addEventListener('click', e => {
  state = !state;

  document.querySelectorAll('circle').forEach((n, i, a) => {
    setTimeout(() => {
      n.classList.toggle('st0', state);
      e.target.disabled = (state ? a.length - i : i + 1) !== a.length;
    }, (state ? a.length - i : i) * 3);
  });
});

let state = true;
const button = document.querySelector('button');
const circles = document.querySelectorAll('circle');

button.addEventListener('click', () => {
  state = !state;
  toggleCircle(0);
});

function toggleCircle(i) {
  if (i < circles.length) {
    setTimeout(() => {
      const index = state ? circles.length - i - 1 : i;
      circles[index].classList.toggle('st0', state);
      button.disabled = i !== circles.length - 1;
      toggleCircle(i + 1);
    }, 3);
  }
}

let state = true;
const circles = document.querySelectorAll('circle');

document.querySelector('button').addEventListener('click', function() {
  this.disabled = true;
  state = !state;

  let i = 0;
  const intervalID = setInterval(() => {
    const index = state ? circles.length - i - 1 : i;
    circles[index].classList.toggle('st0', state);
    if (++i === circles.length) {
      this.disabled = false;
      clearInterval(intervalID);
    }
  }, 3);
});

moment.utc(moment.duration(endTime) - moment.duration(startTime)).format('HH:mm')

var delayAnim = 4;
<...>
'animation-delay': '.' + delayAnim + 's'