I was wondering what prime numbers would look like in binary format:

function isPrime(n) {
  if (n < 2) return false;
  if (n < 4) return true;
  if (n % 2 === 0) return false;
  for (var i = 3; i <= Math.sqrt(n); i += 2)
    if (n % i === 0) return false;
  return true;
}

function int2bin(n) {
  return n.toString(2).split('').map(function(c) { return c === '1' ? '#' : '.'; }).join(' ');
}

function centerLine(s, w) {
  var pad = Math.floor((w - s.length) / 2);
  return pad > 0 ? ' '.repeat(pad) + s : s;
}

var lines = [], prevBits = 0, width = 15;
for (var n = 2; n <= 255; n++) {
  if (isPrime(n)) {
    var bits = n.toString(2).length;
    if (prevBits !== 0 && bits !== prevBits) lines.push('');
    lines.push(centerLine(int2bin(n), width));
    prevBits = bits;
  }
}

Screenshot loaded in Photoshop with a “Chalk & Charcoal” filter.