问无论深度如何，jquery都是第一个子级

EN

$('div:first-child').css({border: '1px solid'});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div>
  <div>I am a first child</div>
  <div>I am a second child
    <div> I am a nested first child</div>
    <div> I am a nested second child</div>
  </div>
</div>
<div>Yet another div child of the body</div>

var children = $('.childclass');
console.log(children.length)//returns "3".
$(children[1]) //returns the second "childclass" instance
$(children[1]).css('color', 'red'); //will give the 2nd instance of the childclass red text.

<div class="topclass">
  <div class="childclass">
      <div class="someotherclass"></div>
      <div class="childclass"></div>
   </div>
   <div class="childclass"></div>
</div>