Square

function zoom(n) {
  const length = n % 2 ? n : n - 1;
  const center = (length - 1) / 2;

  if (length < 0) {
    return '';
  }

  function getSquare(square, level, symbol) {
    if (level * 2 > length) {
      return square.join('\n');
    }

    const newSymbol = symbol === '□' ? '■' : '□';

    if (level === 0) {
      square[center] = symbol;
      return getSquare(square, level + 1, newSymbol);
    }

    const topAndBottomRow = new Array(level * 2 + 2).join(symbol);

    const topRowIndex = center - level;
    const bottomRowIndex = center + level;

    square[topRowIndex] = topAndBottomRow;
    square[bottomRowIndex] = topAndBottomRow;

    Array.from({ length: bottomRowIndex - topRowIndex - 1 }, (_, i) => {
      const index = topRowIndex + i + 1;
      square[index] = `${symbol}${square[index]}${symbol}`;
    });

    return getSquare(square, level + 1, newSymbol);
  }

  return getSquare(new Array(length), 0, '■');
}

console.log(zoom(100));