SOURCE

$(document).on('blur', 'input', blurFn);
function blurFn(e) {
  var cur = $(this);
  var val = cur.val();
  if(val.length < 10) {
    waring('位数少于10！');
  }
}
function waring(msg) {
  $('.waring-bar').html(msg).removeClass('hidden');
}

<input />
<div class="waring-bar hidden"></div>

.hidden{
  display: none;
}
.waring-bar{
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  background: rgba(0,0,0,.2);
}