小编kk *_* kk的帖子

$('.box2').addClass(function(){
  return 'list' + $(this).find('.box1 li').length;
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<ul class="box1">
  <li>a</li>
  <li>b</li>
  <li>c</li>
</ul>


<div class="box2">text</div>

// ADD , Remove input
var counter = 1,
  custom = $('#custom');
$(function() {
  $('#add_field').click(function() {
    counter += 1;
    var newRow = $('<div class="row' + counter + '"><span class="wrap_input"><input id="exception_' + counter + '" name="" type="text"><button class="btn_clear">clear</button><button class="remove-text-box">Remove</button></span></div>');
    custom.append(newRow);
    (function(index) {
      newRow.find('.remove-text-box').click(function() {
        custom.find('.row' + index).remove();
      });
    })(counter);
  });
});
 
// clear input value 
$('.wrap_input').each(function() {
	var $inp = $(this).find("input"),
      $cle = $(this).find(".btn_clear");
	$inp.on("input", function(){
  	$cle.toggle(!!this.value);
  });
	$cle.on("touchstart click", function(e) {
  	e.preventDefault();
    $inp.val("").trigger("input").focus();
    $inp.change();
  });
});

.btn_clear { …