Question

How do I use jQuery to delete all the elements under “li_1” (but not delete li_1)

<ul id="ul_1">
   <li id="li_1">1.1    //want to delete all its child elements
      <ul id="ul_1.1">    
          <li id="li_1.1.1">1.1.1</li>
          <li id="li_1.1.2">1.1.2
              <ul id="ul_1.1.2">
                 <li id="li_1.1.2.1">1.1.2.1</li>
                 <li id="li_1.1.2.2">1.1.2.2</li>
              </ul>
          </li> 
      </ul>
   </li>
   <li id="li_2">1.2</li>
   <li id="li_3">1.3</li>
</ul>

Answer 1

Your markup is incorrect, You should change your markup something like below,

<ul id="ul_1">
   <li id="li_1">1.1    //want to delete all its child elements
      <ul id="ul_1.1">    
          <li id="li_1.1.1">1.1.1</li>
          <li id="li_1.1.2">1.1.2</li>
              <ul id="ul_1.1.2">
                 <li id="li_1.1.2.1">1.1.2.1</li>
                 <li id="li_1.1.2.2">1.1.2.2</li>
              </ul>
      </ul>
   </li>               // <-- Moved your sublist #ul_1.1 inside the li #li_1
   <li id="li_2">1.2</li>
   <li id="li_3">1.3</li>
</ul>

And change your JS as,

$('#li_1 ul').remove();