如何才能将字符串限制在12个字符以内

function dash(el, after) {
  after = after || 3;
  var v = el.value.replace(/[^\dA-Z]/g, ''),
    reg = new RegExp(".{" + after + "}", "g")
  el.value = v.replace(reg, function(a, b, c) {
    return a + '-';
  });
}

var el = document.getElementById('phone');
el.addEventListener('keyup', function() {
  if (el.value.length <= 8) {
    dash(this, 3);
  } else {
    el.value.length++;
  }
  if (el.value.length >= 12) {
    el.value.length = el.substring(0, 12);
  }
});

<span>Phone</span><input id="phone" class="comInputc" name="phone" type="text">