0xD34F

• Производится попытка установить в качестве текущего слайд номер 14. Что делать? Очевидно, установить в качестве текущего четвёртый - остаток от деления изначально указанного номера на количество слайдов.
  
• Как установить минус второй (-2) слайд? - очевидно, это второй с конца, надо добавить количество слайдов, чтобы получить реальный номер.
  
• Зачем одновременно добавляется количество слайдов и берётся остаток? - чтобы один и тот же код корректно обрабатывал переходы из конца в начало и из начала в конец.
  

const WIDTH = 300;
const HEIGHT = 300;
const canvas = document.getElementById('canvas');
canvas.width = WIDTH;
canvas.height = HEIGHT;

const ctx = canvas.getContext('2d');

const
  X = 100,
  Y = 100,
  A = 90,
  B = 50;

const
  N = 1000,
  SIDE = 4,
  HALF_SIDE = SIDE / 2;

const rand = (min, max) => Math.random() * (max - min) + min;

ctx.beginPath();
ctx.ellipse(X, Y, B, A, Math.PI / 2, 0, 2 * Math.PI);
ctx.stroke();

for (let i = 0; i < N; i++) {
  const
    x = rand(-A + HALF_SIDE, A - HALF_SIDE),
    maxAbsY = (B - HALF_SIDE) / A * Math.sqrt(A * A - x * x),
    y = rand(-maxAbsY, maxAbsY);

  ctx.beginPath();
  ctx.rect(x + X - HALF_SIDE, y + Y - HALF_SIDE, SIDE, SIDE);
  ctx.stroke();
}

button.addEventListener('click', () => {
  anotherButton.click();
});

button.addEventListener('click', () => {
  anotherButton.dispatchEvent(new Event('click', { bubbles: true }));
});

function pyramid(n) {
  const result = [];

  for (let i = 0; ++i <= n;) {
    const x = [];
    for (let j = 0; ++j <= i; x.push(1)) ;
    result.push(x);
  }

  return result;
}

const pyramid = length => Array.from({ length }, (_, i) => Array(i + 1).fill(1));

const filterObject = (obj, values) =>
  Object.entries(obj).reduce((acc, [ k, v ]) => (
    values.indexOf(v) !== -1 && (acc[k] = v),
    acc
  ), {});

const result = filterObject(a, b);

const filterObject = (obj, filter) =>
  Object.fromEntries(Object.entries(obj).filter(n => filter(...n)));

const result = filterObject(a, (k, v) => b.includes(v));

return this.$store.authInfo

Объект FileReader позволяет веб-приложениям асинхронно читать содержимое файлов

onFileChange({ target: { files: [ file ] } }) {
  if (file) {
    const reader = new FileReader();
    reader.addEventListener('load', e => {
      this.fileinput = e.target.result;
      console.log(this.fileinput);
    });
    reader.readAsText(file);
  }
},

const flat = arr =>
  (arr || []).reduce((acc, { children, ...n }) => {
    if (n.isExpanded || !children) {
      acc.push(n, ...flat(children));
    } else {
      acc.push({ ...n, children });
    }

    return acc;
  }, []);

const flat = arr =>
  (arr || []).reduce((acc, { children, ...n }) => {
    const flatChildren = flat(children);

    if (n.isExpanded || !children) {
      acc.push(n, ...flatChildren);
    } else {
      acc.push({ ...n, children: flatChildren });
    }

    return acc;
  }, []);

:class="form.FirstName !== null ? form.FirstName ? 'input-successes' : 'input-error' : ''"

:class="{ 'input-successes': form.FirstName, 'input-error': form.FirstName === false }"

:class="form.FirstName !== null && [ 'input-error', 'input-successes' ][+form.FirstName]"

:class="({ true: 'input-successes', false: 'input-error' })[form.FirstName]"

this.update = () => {
  const w = this.w / 2;
  const h = this.h / 2;

  this.x = Math.max(w, Math.min(W - w, this.x + this.dx));
  this.y = Math.max(h, Math.min(H - h, this.y + this.dy));
};

document.addEventListener('keydown', e => {
  switch (e.key) {
    case 'ArrowLeft':  pl.dx = -pl.speed; break;
    case 'ArrowRight': pl.dx =  pl.speed; break;
    case 'ArrowUp':    pl.dy = -pl.speed; break;
    case 'ArrowDown':  pl.dy =  pl.speed; break;
  }
});

document.addEventListener('keyup', e => {
  switch (e.key) {
    case 'ArrowLeft':
    case 'ArrowRight': pl.dx = 0; break;
    case 'ArrowUp':
    case 'ArrowDown':  pl.dy = 0; break;
  }
});

state = {
  likes: localStorage.getItem('likes') | 0,
}

plus = () => {
  this.setState(({ likes }) => ({
    likes: likes + 1,
  }), () => localStorage.setItem('likes', this.state.likes));
}

const tableId = 'news';
const trSelector = 'tbody tr';

$(`#${tableId}`).show().not(`:has(${trSelector})`).hide();

// или

const $table = $('[id="' + tableId + '"]');
$table.toggle(!!$table.find(trSelector).length);

(t => t.hidden = !t.querySelector(trSelector))
(document.querySelector('#'.concat(tableId)));

// или (в стили надо будет добавить .hidden { display: none; })

const table = document.getElementById(tableId);
table.classList.toggle('hidden', Array.prototype.every.call(
  table.tBodies,
  n => !n.rows.length
));

#news:not(:has(tbody tr)) {
  display: none;
}

const attacks = [
  { minChance: 7, damage: 40, name: 'critical' },
  { minChance: 5, damage: 20, name: 'big' },
  { minChance: 0, damage: 10, name: 'weak' },
];

const messages = {
  start: (player, enemy) => `Welcome! Yor health is - ${player}%, your enemy health - ${enemy}%`,
  end: (player, enemy) => `You ${enemy <= 0 ? 'win' : 'lost'}! Your hp level - ${player}, opponents hp level - ${enemy}`,
  chance: (player, enemy) => `your chance - ${player}, your opponent chance - ${enemy}`,
  turn: (player, enemy, hit, isEnemy) =>
`${isEnemy ? 'Enemy' : 'Your'} turn...
${isEnemy ? 'Enemy' : 'You'} did a ${hit} hit
${isEnemy ? 'Your' : 'Enemy'} hp - ${isEnemy ? player : enemy}`,
};


const simpleFight = () => {
  const hp = [ 100, 100 ];

  console.log(messages.start(...hp));

  while (hp.every(n => n > 0)) {
    const chances = hp.map(() => Math.random() * 11 | 0);

    console.log(messages.chance(...chances));

    if (chances[0] !== chances[1]) {
      const chance = Math.max(...chances);
      const attack = attacks.find(n => n.minChance < chance);
      const isEnemyAttacks = chance === chances[1];
      hp[+!isEnemyAttacks] -= attack.damage;

      console.log(messages.turn(...hp, attack.name, isEnemyAttacks));
    }
  }

  console.log(messages.end(...hp));
};

const sum = str.match(/\d+/g).reduce((acc, n) => acc + +n, 0);

// или

let sum = 0;
for (const n of str.split(/\D+/)) {
  sum += Number(n);
}

// или

const sum = eval(str.replace(/\|(\d+)\|/g, '+$1'));

preg_match_all('/\d+/', $str, $matches);
$sum = array_sum($matches[0]);

// или

$sum = 0;
foreach (preg_split('/\D+/', $str) as $n) {
  $sum += intval($n);
}

// или

eval('$sum = '.preg_replace('/\|(\d+)\|/', '+$1', $str).';');

id="mse2_ms|price_0"