2048.js

function rand(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min)) + min; 
}

function placeRandom(field) {
  field[rand(0, 16)] = rand(0, 100) > 50 ? 2 : 4;
}

function create(pattern = null) {
  if (pattern != null) {
    return pattern
      .split('\n')
      .filter(x => x !== '')
      .join('|')
      .split('|')
      .map(x => x === '*' ? null : x);
  }

  const field = new Array(16).fill(null);

  placeRandom(field);
  placeRandom(field);

  return field;
}

function getCol(index) {
  return (index + 4) % 4;
}

function getRow(index) {
  return Math.floor(index / 4);
}

function render(field) {
  return [...field]
    .reduce((map, col, index) => {
      if (getCol(index) === 0) {
        map.push([ ]);
      }

      map[map.length - 1].push(col);

      return map;
    }, [ ])
    .map(row => row
      .map(col => col == null ? '*' : col)
      .join('|'))
    .join('\n');
}

function isFits(x) {
  return x > 0 && x < 4;
}

function isBusy(field, position) {
  return field[position] != null;
}

function moveUp(field) {
  const map = [ ...field ];

  for (let index = 0; index < map.length; index++) {
    if (map[index] == null) {
      continue;
    }

    let pos = index + 0;
    let col = getCol(pos);
    let row = getRow(pos);

    while(isFits(row)) {
      pos -= 4;
      row = getRow(pos);
    }


    if (pos !== index) {
      if (map[pos] === map[index]) {
        map[pos] = map[index] * 2;
      } else if (isBusy(map[pos])) {
        
      } else {
        map[pos] = map[index];
      }

      map[index] = null;
    }
  }

  return map;
}

const map = create(`
*|*|*|*
*|*|*|4
*|*|*|*
*|*|*|2
`);

console.log(render(map));
console.log(render(moveUp(map)));